don't emulate VIOL in error messages

While it might seem like a good idea to "grade" errors by severity,
we are not *really* in any place to do so accurately. Our tasks are

 (a) to decide, internally, whether to print a message or silently
     ignore a malformation, and
 (b) to ultimately judge the file valid or invalid as a whole.

Note that the latter part, as stated before, is not the responsibility
of parse_xrefs().

Reinstate the input file name in these error messages. That information
is useful when running the program on multiple files from a script, as
we have been doing.

While we're at it, fix style (line lengths).