maint: Need robust handling when external source is no longer available

[darcywong00]

I think this is a good interim stop-gap. I am concerned that we will not see the warnings in the logs which may lead to bigger issues in the future, so we need to have a tracking issue for a deeper resolution (which involves caching the files in-repo at submission time). Perhaps this patch should not 'fix' the reporting issue

Originally posted by @mcdurdin in #337 (review)

This comes out of the build currently failing because the external source files for fv.bea.tsaadane are missing.

lexical-models/release/fv/fv.bea.tsaadane/external_source

Lines 1 to 3 in f628994

	fv.bea.tsaadane.model_info https://static.firstvoices.io/keyboards/lexical-models/fv.bea.tsaadane/fv.bea.tsaadane.model_info 3148a5b03ab2f645159d578de556a51e9a2f85dcd5e5e19010714a162f3ac4e1
	fv.bea.tsaadane.model.kmp https://static.firstvoices.io/keyboards/lexical-models/fv.bea.tsaadane/fv.bea.tsaadane.model.kmp b44db9d753f31e5434d6f61c129b9fc2b7672553b959dee00f90991fdd61bfaa
	fv.bea.tsaadane.model.js https://static.firstvoices.io/keyboards/lexical-models/fv.bea.tsaadane/fv.bea.tsaadane.model.js 0ad792674ee8ff752aa9c353e88c1a4013b17bbb35636759fcca494d47886442

Note: retrieve_external_source_model() doesn't have any caching.

[mcdurdin]

Dup of:

• feat(developer): build caching framework for remote resources in keyboards and lexical-models keyman#15545