show more context for errors from fluxctl

[2opremio]

Errors in fluxctl are unwrapped before printing since we want to
format flux-specific errors specially, and they can come wrapped.

However, the unwrapping can lead to losing context when printing
non-flux-specific errors.

For instance, after
https://github.com/justinbarrick/go-k8s-portforward/pulls/3 we display
the following when a kubeconfig file isn't found:

Error: invalid configuration: no configuration has been provided
Run 'fluxctl identity --help' for usage.

Which is the cause of a much more helpful error when in full:

Error: Could not load kubernetes configuration file: invalid configuration: no configuration has been provided
Run 'fluxctl identity --help' for usage.

This change removes the unwrapping for non-flux-specific errors.

[squaremo]

This changes the behaviour when there's a *fluxerr.Error -- it'll print the help text, then the error message, then the usage, while before it just printed the help text. I don't think this is wrong, just flagging it up. This is what it looks like:

$ FLUX_URL=https://cloud.weave.works/api/flux fluxctl identity
== Error ==

The request failed authentication

This most likely means you have a missing or incorrect token. Please
make sure you supply a service token, either by setting the
environment variable FLUX_SERVICE_TOKEN, or using the argument --token
with fluxctl.


Error: executing HTTP request: request failed authentication
Run 'fluxctl identity --help' for usage.

[squaremo]

Makes things better 👍

[2opremio]

This is what it looks like:

$ FLUX_URL=https://cloud.weave.works/api/flux fluxctl identity
== Error ==

The request failed authentication

This most likely means you have a missing or incorrect token. Please
make sure you supply a service token, either by setting the
environment variable FLUX_SERVICE_TOKEN, or using the argument --token
with fluxctl.



Error: executing HTTP request: request failed authentication
Run 'fluxctl identity --help' for usage.

My intention was to unify the error output, but in hindsight it just provides redundant information, so I reverted to the original behavior.