Using error level causes resulting in 'Unknown cause' Exception

Problem/Motivation

Using PAUSE_CAUSE_ERROR_LVL_[X] as cause when pausing the ync client, results in an Exception ('Unknown cause') in the SyncClientPause constructor.
Currently only PAUSE_CAUSE_FORCED, PAUSE_CAUSE_MAINTENANCE, PAUSE_CAUSE_MANUAL are allowed.

Steps to reproduce

• Use the sync client from the sync_client_example module and do a request that results in a 504 status code.
• Process the queue will result in 'Exception: Unknown cause: sync_error_lvl_1'.

Proposed resolution

• Move the PAUSE_CAUSE_ERROR_LVL_[X] constants from SyncClientInterface to the SyncClientPauseInterface
• Update code where these constants are used
• Update SyncClientPause constructor to also allow PAUSE_CAUSE_ERROR_LVL_[X]

Remaining tasks

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