Prefix CLI errors with `cargo:`

[jyn514]

Problem

When using cargo run, it can be hard to distinguish output from cargo and output from the spawned executable, especially if both are using clap. Take for example this error:

$ cargo r --exe
error: unexpected argument '--exe' found

  tip: to pass '--exe' as a value, use '-- --exe'

Usage: run --features <FEATURES> [ARGS]...

For more information, try '--help'.

and compare it to this one:

$ cargo r -- --exe
error: a value is required for '--exe <filename>' but none was supplied

For more information, try '--help'.

both of these look similar, and it is hard to tell what's happening at a glance.

Proposed Solution

Prefix each error: diagnostic with cargo:; the example above would start with cargo: error: unexpected argument '--exe' found

Notes

maybe it sense to do this for everyone upstream in clap instead of just for cargo?

[epage]

imo the usage in the first example should be what differentiates these. Its missing cargo because of cargo's alias system and thats a bug to fix on its own.