This change standardizes threading formatter error handling and moves
the remaining error calls to be warnings instead.
This is in line with already existing code - in most cases warnings were
raised, only a few cases raised errors. These cases do not differ
significantly from other cases in which warnings are raised.
This also fixes GH-692, in which misformatted lines prevent future file
parsing.
This commit also moves the FailWarn method that is used by both the
config and the ascii reader up to the ReaderBackend. Furthermore it
makes the Warning method of ReaderBackend respect the warning
suppression that is introduced by the FailWarn method.
The changes are now a bit more succinct with less code changes required.
Behavior is tested a little bit more thoroughly and a memory problem
when reading incomplete lines was fixed. ReadHeader also always directly
returns if header reading failed.
Error messages now are back to what they were before the change, if the
new behavior is not used.
I also tweaked the documentation text a bit.
