Improve warning and docs on schema export issues #3137

Job	Run time
setup / Compile sqlite3 for tests (ubuntu-latest)	5s
setup / Get and cache Dart dependencies	29s
setup / Compile sqlite3 for tests (macos-latest)	8s
setup / Compile sqlite3 for tests (windows-latest)	38s
drift package	2m 14s
sqlparser	1m 10s
drift_dev	6m 41s
Documentation	5m 7s
Integration tests	3m 0s
test_flutter (3.24)	2m 5s
test_flutter (3.27)	3m 18s
Integration tests for migration tooling	1m 6s
Web integration tests	3m 49s
drift_web_ddc	3m 55s
Drift web unit tests, dart2wasm, chrome	4m 9s
Drift web unit tests, dart2wasm, firefox	4m 24s
	42m 18s

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