Fix wrong log message when a promise is ignored due to 'ifvarclass' not matching

This situation occurs if a promise is prefixed with a "class_exp_1::" notation
and has a "ifvarclass => "class_exp_2" statement. Then the log message
says:
Skipping next promise '<promise>', as context 'class_exp_1' is not relevant
Whereas it should say:
Skipping next promise '<promise>', as var-context 'class_exp_2' is not relevant