Auto merge of #5061 - ehuss:git-error-auth, r=alexcrichton

Display git errors during authentication.

Certain git errors during authentication were being converted to internal errors which meant that they are only seen if you pass `--verbose`.  This may not be obvious, and many of these messages are helpful for diagnosing git errors.  This change makes these errors always be displayed.

Fixes #5035.

Note: Some of the git errors are currently unhelpful.  Once Cargo has updated git2-rs to include alexcrichton/git2-rs#298, these errors will improve.  (@alexcrichton, I can make a PR to update Cargo for the changes in git2 if you'd like).

I'm uncertain if this is a good solution, since the error messages in some cases are a little verbose (such as showing `class=...`).  Here is a sample of what some of the messages look like:

<details><summary>Error Message Examples</summary>
<p>
Example of the git message shown below the "attempted yadda yadda" message.

Scenario | Message
---------|--------
No ssh-agent, multiple usernames | `error authenticating: ; class=Ssh (23)`
| | †`error authenticating: no auth sock variable; class=Ssh (23)`
No ssh-agent, one username | `an unknown git error occurred`
| | †`error authenticating: no auth sock variable; class=Ssh (23)`
Incorrect ssh-agent setup | `error authenticating: failed connecting with agent; class=Ssh (23)`
ssh-agent no keys, one username | `an unknown git error occurred`
| | †`failed to acquire username/password from local configuration`
ssh-agent no keys, multiple usernames | `error authenticating: ; class=Ssh (23)`
| | †`no authentication available`
auth success, bad path | `fatal: '/path/to/repo/' does not appear to be a git repository; class=Ssh (23); code=Eof (-20)`
| | ‡`ERROR: Repository not found.; class=Ssh (23); code=Eof (-20)`
bad username | `an unknown git error occurred`
| | †`failed to acquire username/password from local configuration`
| | ‡`error authenticating: Username/PublicKey combination invalid; class=Ssh (23)`

† - Messages once git2-rs is updated.
‡ - Github message

</p>
</details>