emul: sbs_gauge: Return err or unsupported feature

The fuel gauge emulators will attempt to access a 0 pointer when running a
backend emulator function that hasn't been implemented.

Add an explicit runtime check to return -ENOTSUP to signify the emulator
feature is not supported. We do not ASSERT here so we can write tests that
are generic and can run with various emulators that support a variety of
features.

Signed-off-by: Aaron Massey <aaronmassey@google.com>