provide workaround error msg if failed to load external module from VFS

Author: tomasstupka

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/common/CExtContext.java

@@ -314,6 +314,14 @@ public static Object loadCExtModule(Node location, PythonContext context, Module
             } catch (PException e) {
                 throw e;
             } catch (AbstractTruffleException e) {
+                if (!realPath.exists() && realPath.toString().contains("org.graalvm.python.vfsx")) {
+                    // file does not exist and it is from VirtualFileSystem
+                    // => we probably failed to extract it due to unconventional libs location
+                    getLogger().severe(String.format("could not load module %s (real path: %s) from virtual file system.\n\n" +
+                                    "!!! Please try to run with java system property graalpy.vfs.extractOnStartup=true !!!\n", spec.path, realPath));
+
+                }
+
                 throw new ImportException(CExtContext.wrapJavaException(e, location), spec.name, spec.path, ErrorMessages.CANNOT_LOAD_M, spec.path, e);
             }
         } else {

graalpython/org.graalvm.python.embedding/src/org/graalvm/python/embedding/utils/VirtualFileSystem.java

@@ -364,7 +364,7 @@ private VirtualFileSystem(Predicate<Path> extractFilter,
         this.extractFilter = extractFilter;
         if (extractFilter != null) {
             try {
-                this.extractDir = Files.createTempDirectory("vfsx");
+                this.extractDir = Files.createTempDirectory("org.graalvm.python.vfsx");
                 this.deleteTempDir = new DeleteTempDir(this.extractDir);
                 Runtime.getRuntime().addShutdownHook(deleteTempDir);
             } catch (IOException e) {

graalpython/org.graalvm.python.embedding/src/org/graalvm/python/embedding/vfs/VirtualFileSystem.java

@@ -360,7 +360,7 @@ private VirtualFileSystem(Predicate<Path> extractFilter,
         this.extractFilter = extractFilter;
         if (extractFilter != null) {
             try {
-                this.extractDir = Files.createTempDirectory("vfsx");
+                this.extractDir = Files.createTempDirectory("org.graalvm.python.vfsx");
                 this.deleteTempDir = new DeleteTempDir(this.extractDir);
                 Runtime.getRuntime().addShutdownHook(deleteTempDir);
             } catch (IOException e) {