Fixed: attempting to use an unsupported built in image in IB would generate an error, but not explain why.

Previously, attempting to use an unsupported built in image such as NSActionTemplate would generate an error message that said, "The image “NSActionTemplate” cannot be found." This would lead the user to think that there was something wrong with nib2cib, instead of understanding that only certain built in images are supported (currently NSAddTemplate and NSRemoveTemplate).

With this commit, an attempt to use an unsupported built in image (where built in is assumed to be any name that matches /^NS[A-Z][A-Za-z]+$/) results in the error message "The built in image “<name>” is not supported."