[native_assets_cli] Yaml parsing should throw FormatExceptions

[dcharkes]

Currently, the fromYaml factory constructors are expecting mostly well formatted yaml contents, and will fail on casts rather than throw FormatExceptions in a bunch of cases.

If other implementations of build.dart do not use the toYaml provided in this package, then this could potentially make the native_assets_builder fail in dartdev/flutter_tools.

We should program more defensively and throw FormatExceptions everywhere.