Clarify which errors are format string errors

There were a few ambiguous error messages which look like they could have
cropped up from either the rust compiler for the format string parser. To
differentiate, the prefix 'invalid format string' is now added in front of all
format string errors.

cc #9970
auto merge of #10306 : alexcrichton/rust/issue-9970-better, r=huonw

There were a few ambiguous error messages which look like they could have
cropped up from either the rust compiler for the format string parser. To
differentiate, the prefix 'invalid format string' is now added in front of all
format string errors.

cc #9970