error.h: Be more careful with error code masking.

It should never happen that two or more of the three special high bits
(osl, lopsub, system) are set in an integer that stores a paraslash
error value, but gcc-12 can't prove this and complains as follows:

	error.h:304:28: warning: array subscript 268435456 is above array bounds of 'const char * const[220]' [-Warray-bounds]
	  304 |         return para_errlist[num];
	      |                ~~~~~~~~~~~~^~~~~

Avoid this warning by always clearing all three special bits.