Merge pull request #649 from hx2A/fix613

improve error message

Author: hx2A

py5_resources/py5_module/py5/__init__.py

@@ -128,9 +128,16 @@
         print(debug_info, file=sys.stderr)
         raise RuntimeError("py5 is unable to start Java 17 Virtual Machine")
 
-    if JClass("py5.util.CheckHeadless")().test():
+    try:
+        if JClass("py5.util.CheckHeadless")().test():
+            raise RuntimeError(
+                "py5 is unable to run correctly in headless mode. "
+                "Make sure you are running in a graphical environment and that your Java Virtual Machine is not a Headless JVM."
+            )
+    except:
         raise RuntimeError(
-            "py5 is unable to run correctly in headless mode. Make sure you are running in a graphical environment and that your Java Virtual Machine is not a Headless JVM."
+            "Unable to instantiate Java class py5.util.CheckHeadless. "
+            "If you are using PyInstaller right now, please check that all of py5's jar files are included in your package."
         )
 
 import py5_tools.colors.css4 as css4_colors  # noqa