throws exceptions early, catch them late

[fidransky]

Example: SaveDiagram servlet always returns success even though some error occurs in database because SQLExceptions get swallowed right when they are thrown in the DB class.

Follow Throw early, catch late principle to handle errors properly.

[tomasSimandl]

Data access layer throws own exception which is handled with an error page.

@fidransky can you please make a small review if it is correct.

[fidransky]

Awesome! Will do.