Previously tests would not fail if they encountered an error such as
LineOverflow or TrailingWhitespace. Making the tests error out will fix
this mismatch between running rustfmt for real and running the tests.
This also modifies all tests that previously contained errors so that
they no longer contain errors (in almost all of the tests this is
accomplished by setting error_on_line_overflow = false).
We will no longer break in the middle of words, only at whitespace or punctuation.
This means we sometimes over-run, but that seems better than some of the bad splits we see.
Closes#369
