Tweak error message to help debugging.
Tweak error message to help debugging.

Also tweak a docstring.

KEHANG/RMG-Py,pierrelb/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py,faribas/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py