Detect when necessary ERT variables are missing

[perolavsvendsen]

Uploader requires three variables to be set: <SUMO_CASEPATH>, <SUMO_ENV> and <SEARCHPATH>. <SEARCHPATH> is conventionally set in the job call, while <SUMO_CASEPATH> and <SUMO_ENV> are part of configuration.

If the two latter variables are unset, the raw strings will be forwarded to the Uploader. The Uploader will then fail when trying to build a path containing the string "<SUMO_CASEPATH>" and subsequently trying to upload to a Sumo environment called "<SUMO_ENV>".

We should consider detecting this specific situation where these are unset, e.g. check if the input given for "casepath" is a string == "<SUMO_CASEPATH>"