eUICC API error code cleanup.

-Remove result classes in cases that a simple int will do. Split the
int error code space (now shared across all calls) into two parts -
predefined errors (0 or negative) and custom errors (positive).

-Replace "generic error" with "error" in naming.

-Remove detailed code from any result returned by the LPA. There is
still a detailed code in the platform API, which is equal to the
result code returned by the LPA.

Fixes: 38206601
Test: TreeHugger
Change-Id: I304081db2127a22d6dd67b6494b4f3279614d19c