Rustdoc error messages are not consistent

While playing around with rustdoc, trying to find some things out, most errors are not consistent, especially with the new error formatting. Also, since passes are well-defined, listing them can help, especially in an offline scenario.

A small example: (The last one has colours, per the new error format I presume)

$ rustdoc
expected an input file to act on
$ rustdoc --passes
Argument to option 'passes' missing.
$ rustdoc --passes foo src/bar.rs
ERROR:rustdoc: unknown pass foo, skipping
$ rustdoc --passes foo src/non-existing
error: couldn't read "src/bad.rs": No such file or directory (os error 2)

[GuillaumeGomez]

cc @jonathandturner @rust-lang/docs

[sophiajt]

@GuillaumeGomez rustdoc is maintained by the tools team.

cc @rust-lang/tools

[GuillaumeGomez]

@jonathandturner: And by the @rust-lang/docs as well. ;) Thanks for adding the tools team!

@Michael-Zapata: If you have something in mind, just open a PR and I (and others) will help you on it to reach a better outcome.

Closing as solved.