Warning output when backend don't support the provider

> > Since this provider only support beancount output, what happened if I run this?
> > ```shell
> > 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 https://github.com/deb-sig/double-entry-generator/issues/154#issuecomment-2983160171_

Start by running the documented double-entry-generator translate command with the example/hxsec/config.yaml configuration, --provider hxsec, and --target ledger. Trace how provider and target support are checked, then verify that an unsupported combination produces a warning instead of silently generating a misleading Ledger output.

Written by the indexing model from the issue text.