Serializer: name the packet in write errors from a compiled protocol

[u9g]

Problem

Every packet write goes through Serializer.createPacketBuffer, but a compiled protocol never sets e.field, so the errors coming out say nothing about which packet failed:

Error: Write error for undefined : Missing field 'x'
Error: SizeOf error for undefined : 0 is not in the mappings value

The interpreter names it through the switch's field path (params.position.face); the compiler reports no path at all. The error is thrown inside _transform and surfaces asynchronously on the stream, so a caller cannot attribute it after the fact either.

Change

Serializer.createPacketBuffer prefixes the message with in packet <name>: when the value has a name and the error's field path does not already contain it:

in packet position: SizeOf error for undefined : up is not in the mappings value

Interpreted errors and values without a name are untouched.

Testing

• New tests in test/misc.js: compiled gets the prefix, interpreted keeps its field path with no duplicate, a value with no name is left alone.
• npm test: lint clean, 504 passing.