warn if age ratios sum up to more than 1. adressing #778

Better text

Properly handle PostProcessorError exceptions. Adresses #778.
This fix allows showing the postprocessing results even if some
postprocessors failed. For the failed postprocessors, a warning is
inserted in the report.

add test for AgePostProcessor

added tests for genderPostprocessor

File renaming

added minimum needs postprocessor test
Merge pull request #882 from mbernasocchi/inasafe-778

handling nicely post processors errors