This commit adds an internal header `library/mps/error.h` related
to error codes in MPS.
For now, those error codes can be considered internal and thus we
don't have to avoid clashes with other Mbed TLS error codes. This
is OK as long as it's true that MPS isn't public API, and its error
codes are never forwarded to the return values of public API calls.
The error code allocation of MPS will likely need revisiting over time.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
