BaseModel uses mf2005 if specified exe is not found #1896
Description
Describe the bug
If the exe_name provided to child instances of BaseModel (e.g. Modpath7) cannot be resolved, flopy eats the error and falls back to mf2005.
To reproduce
Create and run a MODPATH 7 model, specifying exe_name="mp7" without mp7 available on the PATH.
Expected behavior
It is reasonable to default to mf2005 if no exe is provided. But mf2005 is not a safe assumption for all models deriving from BaseModel. Flopy should make it known if the model run can't be configured to the user's specification.
Additional context
Motivated by #1886