Batch parsing in the UI leads to exception for parse error

Problem/Motivation

\Drupal\l10n_server\ConnectorParseHandlerInterface::parseHandler() documents FALSE as a valid return value.

\Drupal\l10n_server\Form\ConnectorBatchConfirmParseForm::batchOperation(), however, does not account for this and unconditionally calls a method on the return value.

Steps to reproduce

If the setup is such that the parser will return FALSE (for example if \Drupal\l10n_server\Entity\Storage\L10nServerReleaseStorage::getIdsToRefresh() returns an empty set) attempt to initiate a batch parsing job from the UI. This will lead to an exception.

Proposed resolution

Account for the return value being FALSE.

Remaining tasks

User interface changes

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API changes

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Data model changes

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