[entropy.spm] create SpmPlugin.Error base exception class for all SPM issues
[entropy.spm] rework SpmPlugin.execute_package_phase API specifications

This makes phase execution and error handling more fine grained and on the
long run more robust as well.
Any exception raised by SPM is now considered fatal, while phase execution
failures (execution with error code != 0) are still considered non-fatal for now.
Release Entropy 174
Tagging Entropy version 174
Merge remote-tracking branch 'upstream/master'