fix backtrace printed when cannot get lock.
change lock error handling code so exceptions have useful info and
exception handling in one place.
add test case for when cannot get lock.
adding fooadding bombadding a.cadding dir/a.oadding dir/b.o! a.c! dir/a.o! dir/b.o? .hgignorea.c: unsupported file type (type is fifo)! a.c