Fixes couple of minor issues with JP2 errors:
1. The tests passed as false-positives even if regex is changed to arbitary pattern, because the promise returned from `assert.rejects` was ignored and the test ended prematurely. This is fixed by removing `{ ... }` around the test function body.
2. This, in turn, hid an issue with `toFile` not throwing the expected error message which was instead propagating `Error: VipsOperation: class "jp2ksave" not found` from libvips. This is now fixed by manually checking the extension before calling into libvips.
3. Pre-creating error instances like `errJp2Save` did is sometimes tempting, but is problematic for debugging because it hides the actual stacktrace of the error (the stacktrace is collected at the moment of `new Error` creation). This is now turned into a function that creates error with the right stack.
