XSB Src (SVN)-old

processes trying to compile a .P file at the same time. This is not
guaranteed to avoid all kinds of collisions, just simple ones I've encountered
when starting multiple processes at the same time, when they use the same
files that need re-compiling.
2. Added code to catch compiler errors and close the object file. On windows
this lack had sometimes kept corrupted .xwam files from being deleted, and
resulted in confusing results, fixable only by explicitly deleting the .xwam
file. This may only be a windows issue.
3. In looking at compiler errors, I noticed that in the move from written
messages to thrown errors, we lost some possibly useful information in the
resulting error messages, so I added it back in. (And did a couple of
other small improvements in the area of compiler errors.)