panic="xxx" error message is wrong

[rodrigorc]

Problem
The error message when writing an invalid profile.panic option is wrong. It says:

error: incorrect value `xxx` for codegen option `panic` - either `panic` or `abort` was expected

It should say instead:

error: incorrect value `xxx` for codegen option `panic` - either `unwind` or `abort` was expected

That is, valid values for this option are 'unwind' and 'abort', but not 'panic'.

Steps

1. Add to Cargo.toml: [profile] panic = 'xxx'
2. Run cargo build.

Output of cargo version:

cargo 1.32.0-nightly (b3d0b2e54 2018-11-15)

[ehuss]

This is an issue in rustc. I can post a quick fix.