Better error message for borked JAVA_HOME environment variable

And corresponding test

Author: dlenski

test/test_all.py

@@ -162,6 +162,17 @@ def test_bad_classpath():
         test_reader = zxing.BarCodeReader(classpath=mkdtemp())
 
 
+def test_wrong_JAVA_HOME():
+    saved_JAVA_HOME = os.environ.get('JAVA_HOME')
+    try:
+        del os.environ['JAVA_HOME']
+        test_reader = zxing.BarCodeReader()
+        with helper.assertRaises(zxing.BarCodeReaderException):
+            test_reader.decode(test_barcodes[0][0])
+    finally:
+        os.environ['JAVA_HOME'] = saved_JAVA_HOME
+
+
 @with_setup(setup_reader)
 def test_nonexistent_file_error():
     global test_reader

zxing/__init__.py

@@ -140,6 +140,8 @@ def decode(self, filenames, try_harder=False, possible_formats=None, pure_barcod
             raise BarCodeReaderException("Java library could not read image (is it in a supported format?)", fn)
         elif stdout.startswith(b'''Exception '''):
             raise BarCodeReaderException("Unknown Java exception", self.java) from sp.CalledProcessError(0, cmd, stdout)
+        elif stdout.startswith(b'''The operation couldn't be completed. Unable to locate a Java Runtime.'''):
+            raise BarCodeReaderException("Unable to locate Java runtime (check JAVA_HOME variable and other configuration)", self.java) from sp.CalledProcessError(p.returncode, cmd, stdout)
         elif p.returncode:
             raise BarCodeReaderException("Unexpected Java subprocess return code", self.java) from sp.CalledProcessError(p.returncode, cmd, stdout)