iddifff: Provide better error messages when there are format issues

[kesara]

Description

When DOS format text files with (UTF-8 code points) are provided, author tools provide the following error message: TypeError: Node.appendChild: Argument 1 is not an object.

This is caused by awk in rfcdiff:

'usr/bin/awk: towc: multibyte conversion failure on: '�s operating temperature.

 input record number 298, file /tmp/rfcdiff-76592/2/draft-ietf-tvr-use-cases-07.txt
 source line number 2
cmp: EOF on 2/draft-ietf-tvr-use-cases-07.txt after byte 12250

Author Tools should provide a better error message when this occurs.

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