housekeeping: Fix problems and standardise error creation
Replace status.Errorf() with helper.ErrXXXf(): the error functions from the status package should not be used
in the code. Instead, the corresponding helper function needs to be used. It will prevent incorrect error wrapping and pop-up the proper status code for gRPC.
The error wrapping uses an old '%s' format for adding error
context, but with Go1.13 we can use '%w' and unwrap an error
afterwards if needed.
After applying t.Parallel()
to the tests it reduces
time to run the tests on my local. It should reduce
time to run the tests on the CI as well.