hook: don't crash on syntax errors in python hooks We had some real-world cases where syntax errors in Python hooks would crash the whole process and leave it in an indeterminate state. Handle those better.

hook: for python hook exceptions, add note to run with --traceback Just like with ImportErrors, it isn't obvious that --traceback will produce helpful debugging output here.

hook: add tests for failing post- python hooks I couldn't find any tests for this.

hook: even fewer parentheses for load errors Missed this one.

destutil: document various failure cases We document what various conditional branch mean and clarify that they are exclusive (since they all end up in with exception raised).

destutil: consistently retrieve 'p1' and 'branch' We already read p1 from the dirstate so let's read the branch from it too.