TS: Tolerate syntax errors in class declaration

[asger-semmle]

The extractor would crash if given the following invalid TypeScript file:

export class {}

We bail out if the TypeScript compiler reports a syntax error, but in this case, it's not reported as a syntax error, but presumably some other kind of error in a later pass we don't run as part of extraction.

The anonymous class was somewhat optimistically converted to a ClassExpression which led to a crash, as the top-level is expected to contain statements, not expressions.