[MERGE] Better error message

[John Arbash Meinel]

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This is just a drive-by patch I wrote that should give a better error
when there is something wrong with a dirstate file.

I don't have a test case, as it requires creating an explicitly broken
dirstate file, and then wedging in an "iter_changes()" call to it, and
while that would be good, I didn't feel it was really worth the time spent.

John
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