remove "sys" from legend if sys failed
Implements the proposed behavior for systematic error bands:
- Try to retrieve the histogram of a systematic variation
- If this fails, WARN the user and try to get yield-only systematic variation
- If this fails, issue an ERROR message and scale systematic error band to 0
- Write "sys + stat" in the legend if and only if one of the above methods worked
See https://its.cern.ch/jira/projects/HWWATLAS/issues/HWWATLAS-174