lib: avoid negating VLC_EGENERIC error code

`VLC_EGENERIC` is defined to `INT_MIN` and negating as a 32 bits integer
is undefined behavior.
Let's print a custom error message instead, as it will never be a valid
errno value anyway.

Reported by UBSAN:
../../lib/core.c:72:9: runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself