restore: Correct wrong error names in @return doc comments
Convert int16_t macro error types into enum to improve debugging/type-checking

This simple change provides various benefits for developers like compile
time errors, better auto-completition in editors and the ability of a
debugger to show the constant instead of just the raw value.
Thanks to Aaron Burghardt for the proposal.
idevicebackup: Fix error variables for type consistency
idevicebackup2: Fix error variables for type consistency