Crash if directory has been deleted

[Uncommon]

If the directory for an open repository is deleted, GitX crashes with an uncaught exception.

2011-06-10 09:26:43.255 GitX[85144:aa8f] An uncaught exception was raised
2011-06-10 09:26:43.255 GitX[85144:aa8f] working directory doesn't exist.
2011-06-10 09:26:43.256 GitX[85144:aa8f] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'working directory doesn't exist.'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x00007fff85f847b4 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x00007fff85a830f3 objc_exception_throw + 45
    2   CoreFoundation                      0x00007fff85f845d7 +[NSException raise:format:arguments:] + 103
    3   CoreFoundation                      0x00007fff85f84564 +[NSException raise:format:] + 148
    4   Foundation                          0x00007fff8522d14e -[NSConcreteTask launchWithDictionary:] + 608
    5   GitX                                0x00000001000253d6 -[PBGitRevList walkRevisionListWithSpecifier:] + 1462
    6   Foundation                          0x00007fff851aaf29 __NSThread__main__ + 1429
    7   libSystem.B.dylib                   0x00007fff813144f6 _pthread_start + 331
    8   libSystem.B.dylib                   0x00007fff813143a9 thread_start + 13
)
terminate called after throwing an instance of 'NSException'