Warning output when backend don't support the provider

[Triple-Z]

Since this provider only support beancount output, what happened if I run this?

double-entry-generator translate \
  --config ./example/hxsec/config.yaml \
  --provider hxsec \
  --target ledger \
  --output ./example/hxsec/example-hxsec-output.beancount \
  ./example/hxsec/example-hxsec-records.xls

@Triple-Z Will generate a wrong ledger output without any warnings.

I think this PR can be merged, and we should grab a new issue about this for better user experience.

Originally posted by @Triple-Z in #154 (comment)