ErrorDialogGuiTest: Extract exception constant
ErrorDialogGuiTest: Extract exception constant

ErrorDialogGuiTest#showErrorDialogs() tests the handling of the dialog
box launch event by using a custom made exception.

The message of the exception can be taken from the Exception#toString()
method. However, it is not used in the verification stage, which uses a
hard-coded magic string instead.

Let's extract the exception so that it can be reused to test that the
proper error message is shown during the verification stage.

CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,se-edu/addressbook-level3