Fixes missing += in error message construction, consolidates addmodel command.
Fixes missing += in error message construction, consolidates addmodel command.

Baaaaam/cyclus,rwcarlsen/cyclus,rwcarlsen/cyclus,mbmcgarry/cyclus,rwcarlsen/cyclus,rwcarlsen/cyclus,Baaaaam/cyclus,gidden/cyclus,Baaaaam/cyclus,mbmcgarry/cyclus,gidden/cyclus,gidden/cyclus,hodger/cyclus,mbmcgarry/cyclus,hodger/cyclus,mbmcgarry/cyclus,hodger/cyclus,hodger/cyclus,gidden/cyclus,hodger/cyclus