Change new API JS_SetUncatchableError before release

[bnoordhuis]

flag is too cryptic and should have been called something like is_uncatchable, or have been left out and been replaced with:

JS_EXTERN void JS_SetUncatchableError(JSContext *ctx, JSValue val);
JS_EXTERN void JS_ClearUncatchableError(JSContext *ctx, JSValue val);

Originally posted by @bnoordhuis in #824 (comment)