Adjust 4xx error codes

[PropGit]

Adjust some 4xx error codes that relate to internal loader handling for Propeller discovery and communication errors so that the external PropLoader can apply special processing to those cases. This relates to PropLoader Issue #28.

Currently, many Propeller loading errors simply use the code 400 and the PropLoader treats them all the same.

Specifically, some more general errors can keep the 400 coded response, but others, like "Propeller not found,", "Rx Handshake timeout," "Invalid Rx handshake," "Version response timeout," and "Invalid version response," should have unique codes, like 401, 402, 403, etc.

(some of these error messages are made up during this writing, but need to be errors if they don't already exist).

[PropGit]

Resolved by returning an error text message with every 400 result. PropLoader now uses those text messages to do something sensible.

Needs verification.