Show error label in designable view

[stephsharp]

Currently prepareForInterfaceBuilder hides the error label because I was unable to get it to show up correctly in the storyboard.

Steps to fix:

1. Currently the error property is of type NSError, but the localized description is the only thing that's used in MFTextField so it could be changed to a string instead.

2. Making the error an NSString instead of an NSError would allow the property to be made inspectable. The error string could then be set in the storyboard.

3. prepareForInterfaceBuilder could be updated to not remove the error label anymore and call sharedInit instead.

Considerations: This would be a breaking change to the API.