Check fnt files for errors prior to adding to scene

[andrew0]

As it stands, when adding a CCLabelBMFont, if there is an error (e.g. missing image file) the program just crashes as at NSAssert from CCLabelBMFont.m. This should be avoided and instead just display an error message to the user, informing them of the issue.

The easiest way to do this would be to have NSExceptions for CCLabelBMFont instead of NSAsserts. This way the error could simply be caught, then the error message can be relayed to the user.