Occasional exception in cell instantiation is annoying

[ZevEisenberg]

On this line: https://github.com/Raizlabs/RZCellSizeManager/blob/develop/RZCellSizeManager/RZCellSizeManager.m#L446

The exception thrown by the system should be avoidable by checking if the .xib file exists before trying to load it. Otherwise, if you set a breakpoint for all exceptions, it will break every time cell size manager tries to load a prototype cell that doesn't have a xib.