Show rosdep-fix-permissions advice when cache read fails.

[nuclearsandwich]

By re-raising this IOError as a CachePermissionsError the advice to run
rosdep fix-permissions will be displayed instead of the general
stacktrace.

If rosdep update is run as root and then a command like rosdep resolve
is used to read from the cache rather than write to it this stacktrace
occurs as reported in #782.

Before

❯ rosdep resolve python

ERROR: Rosdep experienced an error: [Errno 13] Permission denied: '/home/steven/.ros/rosdep/sources.cache/index'
Please go to the rosdep page [1] and file a bug report with the stack trace below.
[1] : http://www.ros.org/wiki/rosdep

rosdep version: 0.20.0

Traceback (most recent call last):
  File "/home/steven/osrf/rosdep/src/rosdep2/main.py", line 144, in rosdep_main
    exit_code = _rosdep_main(args)
  File "/home/steven/osrf/rosdep/src/rosdep2/main.py", line 426, in _rosdep_main
    return _rosdep_args_handler(command, parser, options, args)
  File "/home/steven/osrf/rosdep/src/rosdep2/main.py", line 448, in _rosdep_args_handler
    return command_handlers[command](args, options)
  File "/home/steven/osrf/rosdep/src/rosdep2/main.py", line 853, in command_resolve
    lookup = _get_default_RosdepLookup(options)
  File "/home/steven/osrf/rosdep/src/rosdep2/main.py", line 132, in _get_default_RosdepLookup
    sources_loader = SourcesListLoader.create_default(sources_cache_dir=options.sources_cache_dir,
  File "/home/steven/osrf/rosdep/src/rosdep2/sources_list.py", line 603, in create_default
    sources = load_cached_sources_list(sources_cache_dir=sources_cache_dir, verbose=verbose)
  File "/home/steven/osrf/rosdep/src/rosdep2/sources_list.py", line 561, in load_cached_sources_list
    with open(cache_index, 'r') as f:
PermissionError: [Errno 13] Permission denied: '/home/steven/.ros/rosdep/sources.cache/index'

After

❯ rosdep resolve python
Failed to write cache file: [Errno 13] Permission denied: '/home/steven/.ros/rosdep/sources.cache/index'
Try running 'sudo rosdep fix-permissions'

[cottsay]

One minor suggestion, but this looks good to me.