Fix errors for missing OpenJPEG (#3442)

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.

lib/output.js

@@ -25,7 +25,9 @@ const formats = new Map([
   ['j2c', 'jp2']
 ]);
 
-const errJp2Save = new Error('JP2 output requires libvips with support for OpenJPEG');
+const jp2Regex = /\.jp[2x]|j2[kc]$/i;
+
+const errJp2Save = () => new Error('JP2 output requires libvips with support for OpenJPEG');
 
 const bitdepthFromColourCount = (colours) => 1 << 31 - Math.clz32(Math.ceil(Math.log2(colours)));
 
@@ -68,6 +70,8 @@ function toFile (fileOut, callback) {
     err = new Error('Missing output file path');
   } else if (is.string(this.options.input.file) && path.resolve(this.options.input.file) === path.resolve(fileOut)) {
     err = new Error('Cannot use same file for input and output');
+  } else if (jp2Regex.test(fileOut) && !this.constructor.format.jp2k.output.file) {
+    err = errJp2Save();
   }
   if (err) {
     if (is.fn(callback)) {
@@ -630,7 +634,7 @@ function gif (options) {
 /* istanbul ignore next */
 function jp2 (options) {
   if (!this.constructor.format.jp2k.output.buffer) {
-    throw errJp2Save;
+    throw errJp2Save();
   }
   if (is.object(options)) {
     if (is.defined(options.quality)) {