Make missing node type error popup non-permanent
Fixes Issue #113
Refinements to Unknown-node handling
Refinement to #113
Handle Buffer objects properly in Serial out node
Fixes #115
The serial out node does a JSON.stringify if the payload is an object. This was incorrectly being applied to Buffer objects, causing the output seen in issue #115.

The Buffer is now passed through as-is (with the newline appended if so configured).