Improve error message when failing to locate images #101

Short overview
As a nut.js user I want to get additional information why image detection failed.

Use case
Easier debugging of failures

Detailed description
Currently, nut.js only reports a failure to detect images with an error message similar to the following:

Failed: Searching for $image failed. Reason: 'Unable to locate $image, no match!'

It would be nice to have more detailed output why detection failed, e.g. confidence values etc.

Additional content

Please provide any (mandatory) additional data for your enhancement