Correct evaluation of xop error codes

[MichaelHuth]

After some testing it turns out that the following way is working properly:

	variable err

	// for first loaded XOP -> xop error codes returned are in the range 10000+ upto max. 10999; error codes -1 to 9999 are Igor Pro error codes
	// for second+ loaded XOP -> xop error codes returned are offsetted by n x 0x10000 per XOP instead of 10000.
	// Therefore, returning the code through RTE and directly through V_flag (SetOperationReturnValue):
	err = err < 0xFFFF ? err : (err & 0xFFFF) + 10000
	// Note: Getting the error message through GetRTErrMessage, GetErrMessage(code) requires the original RTE code (does not work with directly return through  V_flag).

based partly on https://www.wavemetrics.com/forum/general/querying-xop-error-codes

Edited: Added (...) as + has higher precendence than &