Write offending script to stderr (#i530)

[BoPeng]

#1530

[gaow]

************* Module sos.tasks
src/sos/tasks.py:518:29: E0601: Using variable 'result' before assignment (used-before-assignment)
src/sos/tasks.py:520:29: E0601: Using variable 'signature' before assignment (used-before-assignment)
************* Module sos.eval
src/sos/eval.py:511:0: C0303: Trailing whitespace (trailing-whitespace)
************* Module sos.singularity.client
src/sos/singularity/client.py:335:0: W1404: Implicit string concatenation found in list (implicit-str-concat)

These are some issues reported by pylint.