better description of GAMS execution/license errors & debugging

The tutorial and docs should explain how to deal with different types of GAMS errors, like missing/invalid license or execution errors (infeasibility).

The debugging features also need to be explained.

Start by reviewing the tutorial and existing documentation sections covering GAMS execution, licenses, infeasibility, and debugging. Identify where these topics belong and document the relevant error types and debugging features; the work is done when users can find guidance for diagnosing each listed problem.

Written by the indexing model from the issue text.