Improve error message when steady state can not be found

[gustavdelius]

The current error message:

Invalid parameters
These parameter do not lead to an acceptable steady state. Please choose other values.
The error message was:
Error in doTryCatch(return(expr), name, parentenv, handler): With these parameter values the Herring does not have enough food to cover its metabolic cost

should be made more user-friendly by 1) taking out the technical stuff about doTryCatch() and by suggesting what to do to fix the situation.