Return a more verbose error when formatting a file fails
Return a more verbose error when formatting a file fails

Expands the error message returned if the after_file function fails to
also include path_str. This allows users to better identify files that are
not being formatted.

graydon/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt,aidancully/rust,graydon/rust,graydon/rust,SingingTree/rustfmt,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,SingingTree/rustfmt,aidancully/rust,rust-lang/rustfmt