Add logic and message to alert when EEPROM 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 an EEPROM checksum failure message.

• Add fatal error message, "EEPROM checksum failed" and give it the next available error code.
• Add logic to emit that message when an EEPROM checksum response is received and doesn't match the proper value.
  • Note: doesn't apply if there's a fatal timeout waiting for EEPROM checksum response; that's handled by Issue #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
004-Programming EEPROM
102-ERROR: Download failed: -1

we see this (assuming the error code is 125):

001-Opening file '../myfile.binary'
002-Downloading file to port ...
009-... bytes sent
003-Verifying RAM
004-Programming EEPROM
125-ERROR: EEPROM 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]

Verified in v1.0-37. EEPROM Checksum error code is 126.