Raise exception if coverage have no llbbox
Set coverage to False if catched exception
Skip task if coverage is False
Show skipped tasks in summary
Updated test
Removed never reached code
Merge branch 'empty-extent' into seed-configuration-error-handling

Conflicts:
	mapproxy/seed/config.py
Raise GeometryError for shapely can't load geometry
Raise GeometryError for boundless geometries
Reraise exceptions with additional exception message
Raise GeometryError if geometry have no extent
Look for GeometryError instead of SeedConfigurationError
Raise SeedConfigurationError on source reading error
Catch seed coverage TransformationError
Raise EmptyGeometryError if geometry have no bounds
Raise SeedConfigurationError if coverage cannot be read ro geometry broken
Raise EmptyCoverageError in case of empty geometries