[ABW]: added custom error triggered by find_comment_id() to make difference...

[ABW]: added custom error triggered by find_comment_id() to make difference between deleted post and nonexisting one
all API calls that use this SQL function are affected (unfortunately it is not yet used everywhere)
get_content/get_content_replies reimplemented as SQL functions
removed superfluous code from condenser_api
restored mute handling in get_content_replies for consistency with get_content