Improved error message
Improved error message


Former-commit-id: 8f3ea6a14bb91a02769cd9806e170a75f0d4e861 [formerly b513594b3c4374eeb42ddc5b96b24393b4c2755c]
Former-commit-id: 04af8b0ec0b7a554daec6cc447557cb2b7dc8d30
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine