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)) {

test/unit/jp2.js

@@ -8,20 +8,20 @@ const fixtures = require('../fixtures');
 
 describe('JP2 output', () => {
   if (!sharp.format.jp2k.input.buffer) {
-    it('JP2 output should fail due to missing OpenJPEG', () => {
+    it('JP2 output should fail due to missing OpenJPEG', () =>
-      assert.rejects(() =>
+      assert.rejects(async () =>
         sharp(fixtures.inputJpg)
           .jp2()
           .toBuffer(),
       /JP2 output requires libvips with support for OpenJPEG/
-      );
+      )
-    });
+    );
 
-    it('JP2 file output should fail due to missing OpenJPEG', () => {
+    it('JP2 file output should fail due to missing OpenJPEG', () =>
-      assert.rejects(async () => await sharp().toFile('test.jp2'),
+      assert.rejects(async () => sharp(fixtures.inputJpg).toFile('test.jp2'),
         /JP2 output requires libvips with support for OpenJPEG/
-      );
+      )
-    });
+    );
   } else {
     it('JP2 Buffer to PNG Buffer', () => {
       sharp(fs.readFileSync(fixtures.inputJp2))