fix: add an exception if executable not found

[dipinknair]

Currently, if no executable found in default location when using get_mechanical_path , then we are returning None. This is causing issue when we call version_from_path where we give path as input. We do not have any check if path is None.

Alternately we can avoid this exception and return path as empty string (as it is) it is instead of None. If that is the case then version_from_path will take care of value error if path is empty string

[koubaa]

The message of the exception isn't actionable by the end user

[RobPasMue]

LGTM - although we could actively check for None as well. But this is more than enough

[RobPasMue]

We should do a new release after #193 - thanks @nmalsang - what's the status here? We could include this PR as well.

[germa89]

LGTM. But I think the message could be more explicative.

Changes addressed