str::from_bytes should return a Result instead of fail on invalid input.

[carllerche]

It seems to me that returning a Result would be more appropriate than failing.

https://github.com/mozilla/rust/blob/incoming/src/libcore/str.rs#L50

[catamorphism]

See discussion on #1945 -- the right thing to do here would be to use conditions, I think.

[catamorphism]

This should be pretty simple -- working on it.

[catamorphism]

Fixed pending #6920

[catamorphism]

3fc0524 fixes.