Better error handling around repository credentials

[nickmbailey]

There is some room for improvement around the error handling/messaging for repository credentials. For example:

• If gpg is not installed, there is just a "could not run gpg" error rather than a message pointing to the docs (if gpg is installed but misconfigured you get the correct error message).
• If gpg is specified but no credentials.clj.gpg file exists, lein just fails with an auth error rather than pointing to the docs on setting up credentials.clj.gpg
• Similarly for env, if credentials are stored in the environment, but the necessary environment variables aren't present, lein should indicate which environment variables need to be set first.

[hypirion]

Yes, I agree that usability related to GPG is rather bad. Happy to take patches for this.

Some of these fixes may be newbie-friendly.

[maxprokopiev]

I would love to work on that if there are no concerns :)

[hypirion]

@maxprokopiev: Go ahead :)

[devurandom]

I have an issue where my ~/.lein/credentials.clj.gpg contains {#"https://repo.clojars.org" {:username "..." :password "..."}} just as in the example in the deploy docs, but still lein deploy clojars reports: No credentials found for clojars. This error message does not explain why the key #"https://repo.clojars.org" did not match / what string Leiningen tried to match against.