Auto merge of #2551 - alexcrichton:explain, r=brson

Add support for `cargo --explain`

The error messages in the compiler are being tweaked and will likely drop the
`rustc --explain` part of the error message in favor of `--explain`. In that
case you're expected to basically take whatever tool you're using and pass
`--explain` to it with an error code, so let's add it to Cargo as well!