process error with formatting

[cehbrecht]

Overview

This PR updates the app exception ProcessError. Instead of validation and skipping invalid messages it formats the message with only valid characters and length.

Related Issue / Discussion

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• [x ] I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage decreased (-0.2%) to 75.401% when pulling d266900 on cehbrecht:update-process-error into 13d988f on geopython:pywps-4.2.