BUG/MINOR: cfgcond: always set the error string on awslc_api checks

BUG/MINOR: cfgcond: always set the error string on awslc_api checks

Using awslc_api_before() with an invalid argument results in "(null)"
appearing in the error message due to -1 being returned without the
error message being filled. Let's always fill the error message on error.

This was introduced in 3.3 with commit 3d15c07ed0 ("MINOR: cfgcond: add
"awslc_api_atleast" and "awslc_api_before""), and this fix must be
backported to 3.3.