AddEmbeddedImageTest: fully test exceptions for "fail to attach" test cases
AddEmbeddedImageTest: fully test exceptions for "fail to attach" test cases

This commit:
* Adds an `exceptionMessage` index to the `dataFailToAttach()` data provider.
* Renames the `testEmbeddedImageEncodingException()` method to `testFailToAttachException()`.
* Sets the `testFailToAttachException()` method up to use the `dataFailToAttach()` data provider.
* Adds testing of the exception message to the `testFailToAttachException()` method.

With this change, the "fail to attach" test cases are now fully tested for both a `PHPMailer` instance without exceptions enabled, as well as for an instance _with_ exceptions enabled.

PHPMailer/PHPMailer,jimjag/PHPMailer,nawawi/PHPMailer,nawawi/PHPMailer,jimjag/PHPMailer,PHPMailer/PHPMailer