pmerge: handle permission failures in a more friendly way

[4eetah]

Issue #154

[codecov-io]

Current coverage is 55.22% (diff: 18.18%)

Merging #220 into master will decrease coverage by 0.02%

@@             master       #220   diff @@
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  Files           148        148          
  Lines         20908      20918    +10   
  Methods           0          0          
  Messages          0          0          
  Branches       5252       5255     +3   
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+ Hits          11550      11551     +1   
- Misses         8638       8647     +9   
  Partials        720        720          

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[4eetah]

I don't really know if you would like "suppress_exceptions = False" for all triggers. Another way to propagate such exceptions is checking for errno.EACCES in merge engine and having suppress_exceptions enabled:

345                     except Exception as e:
346                         if not trigger.suppress_exceptions:
347                             raise
348
349                         handle = stringio.text_writable()
350                         traceback.print_exc(file=handle)
351
352                         self.observer.warn(
353                             "unhandled exception caught and suppressed:\n%s", handle.getvalue())