Fix mock.go's Int Bool and Error shortcut methods

When using testify, I saw panic: assert: arguments: Error(0) failed because object wasn't correct type: {{ad6e7d77-1745-4761-6c4b-a79ab92a4a0f e6e975ef-3637-4047-436a-018ee9d2de52 %!s(int=17) <nil> <nil> %!s(bool=false) %!s(*string=<nil>) ...
when using the Error(index) function.

This commit simply changes to using %v for printing the unexpected structure in the Bool, Int, and Error shortcut methods.