Show syntax errors

[remy]

I noticed (for a long while) the error messages in syntax weren't being shown. Turns out it was because most error calls don't pass in the file object, so I've used a bit of a blunt tool to fix it and added file as an extern (I hope that's okay).

Example output now when there's an error:

$ ../bin/pmas refurb.asm ../refurb.min ../refurb.sym > /dev/null

Symbol 'REG_EVENT2P' not defined.
refurb.asm:188 'movb [nn+REG_EVENT2P],INT2P_SHOCK'

1 error(s) found.
make: *** [refurb.min] Error 1