Previous to this commit, if the update message had a valid subdomain but
an invalid TXT the error returned was for a bad subdomain. This can
confuse developers who were POSTing junk TXT records to test acme-dns
:-)
This commit adjusts the `webUpdatePost` error handling such that
`!validSubdomain(input)` and `!validTXT(input)` give distinct errors.
The `!validSubdomain` case should never happen in `webUpdatePost`
because `auth.go`'s `Auth` function already vets the post data
subdomain but I retained the error handling code just in case.
Unit tests for an update with an invalid subdomain and an update with an
invalid TXT are included.
This commit fixes the JSON examples in the README. There was a missing
`]` in the example input, a missing IP range in the reflected output,
and a few trailing `,` that shouldn't have been there.
Daniel
