Add JS wrapper for sqlite3_exec() which knows how to handle a JS callback. Add some...

Add JS wrapper for sqlite3_exec() which knows how to handle a JS callback. Add some console.error() reporting of module-load failures, as they otherwise often get silently swallowed up by the loader's mechanisms. Add 'flexible-string' JS-to-WASM argument converter which performs more X-to-string conversions than the 'string' arg converter does.

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