sed: use warn()/err() where appropriate

sed: use warn()/err() where appropriate

Use warn()/err() instead of sed's homegrown warning()/error() for
things other than parser problems.  The warning()/error() functions
display the file and line number in addition to the error message.
This also removes of the COMPILE/FATAL argument to error() since
now all calls to error() are for compilation/parsing issues.
OK op@ espie@