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
Merge pull request #840 from jooooooon/fix_log_message_about_skipping_promises_when_ifvarclass_doesnt_apply

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