Add logic and message to alert when RAM checksum failed

[PropGit]

[This request created in response to Issue #27]

In much the same way that the added "Propeller not found" error message clarifies download failures, we need a RAM checksum failure message.

• Add fatal error message, "RAM checksum failed" and give it the next available error code.
• Add logic to emit that message when a RAM checksum response is received and doesn't match the proper value.
  • Note: doesn't apply if there's a fatal timeout waiting for RAM checksum response; that's handled by Issue Add logic and message to alert when communication lost #39.

This should apply to both wired and wireless downloads and should appear before the "ERROR: Download failed: -1" message so that instead of this:

001-Opening file '../myfile.binary'
002-Downloading file to port ...
009-... bytes sent
003-Verifying RAM
102-ERROR: Download failed: -1

we see this (assuming the error code is 124):

001-Opening file '../myfile.binary'
002-Downloading file to port ...
009-... bytes sent
003-Verifying RAM
124-ERROR: RAM checksum failed
102-ERROR: Download failed: -1

[dbetz]

I've added this to the fast loader code. It is not yet present in the single-stage loader.

[dbetz]

done

[PropGit]

Tested on v1.0-37.