ErrorDialogGuiTest: Use string constants in UiManager
ErrorDialogGuiTest: Use string constants in UiManager

ErrorDialogGuiTest#showErrorDialogs() verifies that the content of the
error dialog box is correct. It uses magic strings to do the
verification.

We already have the appropriate string constants in UiManager.

Let's teach ErrorDialogGuiTest to use the constants in UiManager.

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