prefer error in databroker when multiple algorithms declare the same output & remove some error messages

the error containing "does not produce any data" should not be at this place, should it? unless you want to "force" people to only put real producers into the m_producers rather than a full list of algorithms. And if so, it should not say "IGNORING...", since it actually sets algEntry->dependsOn
also removed the unused algorithm warning since the counting is not yet implemented, and it annoys people currently ^^

@graven FYI

@olupton FYI put the error() instead of warning() for duplicated outputlocations

@sponce FYI: you might want to cherry-pick this and try whether this works out well..