Incorrect naming in std::failure

[brson]

Module should be named panicking to match core, on_fail should be on_panic.

[iKevinY]

Took a shot at this in iKevinY/rust@1e01f7f470f1d79be710d546ab25426fcede59f9, but I wanted to verify here to make sure I interpreted the issue correctly before opening a PR. At a glance, does this look alright? (make check still passes.)

[brson]

@iKevinY Yes looks right. Thanks!