Feature/ogc 1160 formcode warning for empty fieldsets

[Tschuppi81]

Form: Raise error for empty field sets

TYPE: Feature
LINK: ogc-1160

[linear]

OGC-1160 Formcode direkt aufeinanderfolgende Titel

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.15%. Comparing base (a4f76d7) to head (66cc10b).
Report is 2 commits behind head on master.

Additional details and impacted files

Files	Coverage Δ
src/onegov/form/errors.py	88.46% <100.00%> (+1.50%)	⬆️
src/onegov/form/parser/core.py	97.67% <100.00%> (+0.01%)	⬆️
src/onegov/form/parser/form.py	94.08% <100.00%> (ø)
src/onegov/form/validators.py	89.62% <75.00%> (-0.22%)	⬇️

... and 1 file with indirect coverage changes

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[Daverball]

Looks good, but there's a pitfall about the current placement of the check.