raised by the compiler.

XXX For now, text entered into the interactive intepreter is not
printed in the traceback.

Inspired by a patch from Roman Sulzhyk

compile.c:

Add helper fetch_program_text() that opens a file and reads until it
finds the specified line number.  The code is a near duplicate of
similar code in traceback.c.

Modify com_error() to pass two arguments to SyntaxError constructor,
where the second argument contains the offending text when possible.

Modify set_error_location(), now used only by the symtable pass, to
set the text attribute on existing exceptions.

pythonrun.c:

Change parse_syntax_error() to continue of the offset attribute of a
SyntaxError is None.  In this case, it sets offset to -1.

Move code from PyErr_PrintEx() into helper function
print_error_text().  In the helper, only print the caret for a
SyntaxError if offset > 0.