resourceloader: Throw exception when config serialization fails

resourceloader: Throw exception when config serialization fails

If something puts a string that's invalid UTF-8 in a JS config variable,
JSON serialization will fail on the entire config blob. Currently, this
causes the entire config blob to be silently dropped, which breaks all
JavaScript because elementary variables like wgPageName are missing.

This change makes this scenario fail loudly rather than quietly, by
throwing an exception. This also makes bugs like these easier to track
down.

Bug: T206475
Change-Id: Ief2ae00228389a23627d440dc1cd9a54cf2b6926