Improve error message for ValidationError

[lu-fennell]

In addition to the invalid character, the error now also contains a
command_synopsis and argument which identifies the command and
argument that failed to validate.

Fixes #215

Currently the error message just shows a "synopsis" of the command, e.g., FETCH seq query, not the actual arguments. Maybe it makes sense to also show the invalid argument value.. except if it is a secret of course. If you think this is a good idea, and provided this PR goes into the right direction, I can add this functionality also.

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This change is 

[jonhoo]

This is a fantastic change and PR, thank you!

I left two comments inline that I think we should address, but otherwise this is good to land.

[jonhoo]

Excellent, thank you!

[jonhoo]

Now we just need CI to cooperate. I've just pushed a commit to master that should fix some of it. The tarpaulin failure is due to xd009642/tarpaulin#756, which we're still bit by only because we need to update our dependencies. Bumping the currently-outdated dependencies in Cargo.toml should take care of it (though let's do that in a separate PR):

$ cargo outdated -d1
Name              Project  Compat  Latest  Kind    Platform
----              -------  ------  ------  ----    --------
imap-proto        0.14.3   ---     0.15.0  Normal  ---
nom               6.2.1    ---     7.1.0   Normal  ---
ouroboros         0.9.5    ---     0.13.0  Normal  ---
regex             1.4.6    ---     1.5.4   Normal  ---
rustls-connector  0.13.1   ---     0.15.0  Normal  ---

The only change that actually seems related to this PR is the failure of rustfmt on beta. That's optional to fix, though good to do ahead of time now that we know about it.

[jonhoo]

(though please do rebase onto or merge with master just to make sure we didn't miss any regressions)

[lu-fennell]

Ok, done: rebased and fixed formatting.

[jonhoo]

Thanks!