[script][common-arcana] Add low attunement failure to Flag['spell-fail']

[Raykyn55]

My previous fix for this problem wasn't effective because the match for this messaging was being hijacked by the match for You gesture, which was occurring a line above. Backfires and the like are handled with a flag already, adding this messaging should give it the same release treatment.

[buff]>cast
You gesture.
You strain, but are too mentally fatigued to finish the pattern, and it slips away.
>
[buff]>release mana
You aren't harnessing any mana.

[Raykyn55]

resolves https://github.com/rpherbig/dr-scripts/issues/5974