Add error handling to zef_start_config

[SeSodesa]

This is a squashed set of commits. See the individual commit messages below for what was changed. Closes #235.

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zeffiro_interface: change position of zef.start_mode initialization

• The field is now initialized before zef_start_config is called, so that it might be used within it.

• Also ran the MATLAB Smart Indentation tool on the file.

zef_start_config: add error handling to evalc calls

• Now warnings are issued, if the external optimization plugins are not found after their addition to Matlab path has been attempted.

• If Zeffiro was started with a GUI, this warning is given as a blocking error modal window. If the Zeffiro GUI was not initialized, this is a simple non-blocking warning.

zef_start_config: add a warning message function

• The warning messages are now generated with the function warning_message_fn.

• This removes some code repetition.

[SeSodesa]

Looks like there is still a reference to FieldTrip in the CVX warning message. I need to change that.

[SeSodesa]

The messages should be fixed now.