Output weird error message while having bad network issues

[linw1995]

• I have searched the issue tracker and believe that this is not a duplicate.

Steps to reproduce

1. set a wrong proxy configuration to simulate having a bad network issue.
2. run pdm add or pdm lock

export https_proxy=http://127.0.0.1:1111 http_proxy=http://127.0.0.1:1111
pdm add cowsay -v

Actual behavior

Adding packages to default dependencies: cowsay
======== Start resolving requirements ========
	cowsay
	Adding requirement cowsay
🔒 Lock failed
Unable to find a resolution that satisfies the following requirements:
  cowsay (from project)
Please make sure the package names are correct. If so, you can either loosen the version constraints of these dependencies, or set a narrower `requires-python` range in the pyproject.toml.
Traceback (most recent call last):
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/resolver/core.py", line 23, in _merge_into_criterion
    crit = self.state.criteria[name]
KeyError: 'cowsay'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/resolvelib/resolvers.py", line 318, in resolve
    name, crit = self._merge_into_criterion(r, parent=None)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/resolver/core.py", line 25, in _merge_into_criterion
    crit = Criterion.from_requirement(self._p, requirement, parent)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/resolvelib/resolvers.py", line 83, in from_requirement
    raise RequirementsConflicted(criterion)
resolvelib.resolvers.RequirementsConflicted: Requirements conflict: <NamedRequirement cowsay>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/linw1995/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 171, in main
    return Core().main(args)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 133, in main
    raise err.with_traceback(traceback)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 128, in main
    f(options.project, options)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/cli/commands/add.py", line 35, in handle
    actions.do_add(
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/cli/actions.py", line 173, in do_add
    resolved = do_lock(project, strategy, tracked_names, reqs)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/cli/actions.py", line 68, in do_lock
    mapping, dependencies, summaries = resolve(
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/resolver/core.py", line 152, in resolve
    result = resolver.resolve(requirements, max_rounds)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/resolvelib/resolvers.py", line 453, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/Users/linw1995/.local/pipx/venvs/pdm/lib/python3.9/site-packages/resolvelib/resolvers.py", line 320, in resolve
    raise ResolutionImpossible(e.criterion.information)
resolvelib.resolvers.ResolutionImpossible: [RequirementInformation(requirement=<NamedRequirement cowsay>, parent=None)]

Expected behavior

Maybe output a better error message to indicate network issues.

Environment Information

# Paste the output of `pdm info && pdm info --env` below:
PDM version:        1.4.5
Python Interpreter: /Users/linw1995/.pyenv/shims/python (2.7.17)
Project Root:       /private/tmp/pdm_test
{
  "os_name": "posix",
  "python_version": "2.7",
  "platform_system": "Darwin",
  "platform_release": "20.3.0",
  "implementation_version": "0",
  "python_full_version": "2.7.17",
  "implementation_name": "",
  "platform_python_implementation": "CPython",
  "platform_machine": "x86_64",
  "sys_platform": "darwin",
  "platform_version": "Darwin Kernel Version 20.3.0: Thu Jan 21 00:07:06 PST 2021; root:xnu-7195.81.3~1/RELEASE_X86_64"
}

[frostming]

Unfortunately, the real ConnectionError is silenced by pip, so we can't get the exact reason, but to guess from the following 3 possibilities:

1. The package doesn't exist on the PyPI
2. The package index is invalid or down for some reason.
3. Network condition problem.

Fixed on master

[linw1995]

A more meaningful error message would be enough. Thank you!