Replace panic with error message when not declaring a source node in .yml config

[AdamHaffar]

Description

Fixes an issue where the check for len(sourceNodes) in buildNodes() was never true due to a DLQHandlerNode always being appended to nodes resulting in a minimum length of 1 for sourceNodes.

Moves append for DLQHandlerNode to only occur if there are source nodes available. This in turn automatically improves the error message instead of a panic.

Fixes #1629

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[hariso]

@AdamHaffar Thanks for this PR! I think we should handle the case with 0 destination nodes too. I know the original issue is about source nodes, but I believe it's a similar problem for destinations as well.