va: reduce duplication in "Validation result" log message

[jsha]

When we log a validation result with an error, it shows up three places:

• We log the updated challenge object, which includes the error
• We log the error at the top level of the JSON object
• We log the internal error

We should fix this by:

• Not logging the updated challenge object
• Logging the internal error only if it differs from the external error (modulo the prefix of the error type)

[mcpherrinm]

One error that has caused confusion for users is errors of the form

dns :: no valid A records found for example.com; no valid AAAA records found for example.com

I haven't looked at how that's generated, but could we collapse those errors to no valid A or AAAA records found for example.com as a special-case?