Revert "Removed all references to PyslidException for now"

This reverts commit a6c2bfadd960ad3cdc51c65775b33e88e17280ad.
features.has should return a bool except for unexpected exceptions

Conflicts:
	pyslid/features.py
raise PyslidException on errors

Conflicts:
	pyslid/features.py
raise PyslidException instead of returning nothing
Store more infor about caught exceptions
Check whether pyslic returns the correct number of features

Conflicts:
	pyslid/features.py
Correct bug in ClientHelper, add test for failed calculation
Typo whilst fixing conflict in 6d455e222c1a430114b613a9aedd7a75a7626577
Merge pull request #9 from manics/add_exceptions

Add exceptions