Enrich error logging when adding custom configs (rule sets)

[mkcomer]

When setting a custom config, I noticed:

if rule set contents is just "hello" --> we get error
FTL error="yaml: unmarshal errors:\n line 1: cannot unmarshal !!str hello into config.Config"
exit status 1

if rule set contents is "404: Not Found" --> no error, believes it is a valid yaml in general, but it is not a valid rule set yaml --> therefore doesn't add the custom rules and doesn't log this out. None of this information is provided in error logs.

Add enriched error logging when parsing of custom config (rule sets) fails.

I noticed this when designing this story: #1

[mkcomer]

Duplicate issue to inclusive-dev-tools/#17