Warn if user does not use checksum; Error if user have invalid checksum

[chikeichan]

1. If address match checksum, then return NO WARNING, else -->
2. If address is all lower case or all uppercase, then return NO CHECKSUM (yellow warning)
3. If address is mixed case, then return INVALID CHECKSUM (red warning)
   3a. If INVALID CHECKSUM, prevent user from advancing