(ScriptingHandler) Cleanup exception handling

Previously all Exceptions were caught. Now we specialize more cleanly
on ScriptExceptions, by converting to them when needed.

This prevents plugin-wide errors from being caught instead of being reported.
(ScriptingHandler) Separate the list of errors and warnings

Ths allows to keep on the GUI the parsing errors/warning together with the runtime
error when running the script. This also avoids a conversion from ScriptEditorException
to ScriptException which was breaking the message (repeated line and column numbers).
ScriptEditorException is now unneeded.
(ScriptingHandler) Move the code for errors/warnings to its specific class