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 a
adding b
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NONEXISTENT: No such file or directory
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b
a: No such file or directory
NONEXISTENT: No such file or directory
b
a
NONEXISTENT: No such file in rev 14467d15ef43
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