Produce a more useful error message when rustfmt can't be found. Fixes #1205

[luser]

Prior to this change bindgen would simply print any error that occurred
while attempting to run rustfmt straight to stderr using fmt::Debug.
Combined with the fact that rustfmt is enabled by default now this meant
that if rustfmt was missing or not working a cryptic error would be
printed.

Without this patch:

$ PATH= ./target/debug/bindgen /tmp/foo.h
Custom { kind: Other, error: StringError("Cannot find binary path") }
/* automatically generated by rust-bindgen */

With this patch:

$ PATH= ./target/debug/bindgen /tmp/foo.h
Failed to run rustfmt: Cannot find binary path (non-fatal, continuing)
/* automatically generated by rust-bindgen */

I changed the error handling to use fmt::Display instead, since this
is user-visible output, and also added "Failed to run rustfmt:" to make the
source of the error clearer. Additionally since bindgen does not treat this
as fatal I noted that in the output as well.

[emilio]

Thanks!