raise PyslidException instead of returning nothing
raise PyslidException on errors
features.has should return a bool except for unexpected exceptions
Revert "Removed all references to PyslidException for now"

This reverts commit a6c2bfadd960ad3cdc51c65775b33e88e17280ad.
Correct bug in ClientHelper, add test for failed calculation
Store more infor about caught exceptions
Temporarily hard-code the OMERO_CONTENTDB_PATH
temporary additional print statements, override magnification