errors: standardise error formatting

[lrlna]

Description

Align error formatting for info, warn, error logs and structopt invalid commands. This should result in a more cohesive output to the user, and give more visual cues if something goes wrong.

This issue should be tackled after #146, as we need to take advantage of the new Error impl to be able to implement a formatting trait for this issue.

(This issue was initially discussed in internal design document on error standardisation.)

[lrlna]

#186 satisfies this issue.