Improve detection of invalid metadata grabbers.

1) Update GetGrabber to log a message about an invalid grabber.

2) Callers of GetGrabber now check whether a returned grabber is valid
before using it.

mythtv/libs/libmythmetadata/metadatadownload.cpp

@@ -565,6 +565,8 @@ MetadataLookupList MetadataDownload::handleGame(MetadataLookup *lookup)
     MetadataLookupList list;
     MetaGrabberScript grabber =
         MetaGrabberScript::GetGrabber(kGrabberGame, lookup);
+    if (!grabber.IsValid())
+        return {};
 
     // If the inetref is populated, even in kLookupSearch mode,
     // become a kLookupData grab and use that.
@@ -608,6 +610,8 @@ MetadataLookupList MetadataDownload::handleMovie(MetadataLookup *lookup)
 
     MetaGrabberScript grabber =
         MetaGrabberScript::GetGrabber(kGrabberMovie, lookup);
+    if (!grabber.IsValid())
+        return {};
 
     // initial search mode
     if (!lookup->GetInetref().isEmpty() && lookup->GetInetref() != "00000000" &&
@@ -648,6 +652,8 @@ MetadataLookupList MetadataDownload::handleTelevision(MetadataLookup *lookup)
 
     MetaGrabberScript grabber =
         MetaGrabberScript::GetGrabber(kGrabberTelevision, lookup);
+    if (!grabber.IsValid())
+        return {};
     bool searchcollection = false;
 
     // initial search mode

mythtv/libs/libmythmetadata/metadatagrabber.cpp

@@ -145,7 +145,16 @@ MetaGrabberScript MetaGrabberScript::GetGrabber(GrabberType defaultType,
         // fall through
     }
 
-    return GetType(defaultType);
+    auto grabber = GetType(defaultType);
+    if (!grabber.m_valid)
+    {
+        QString name = grabberTypes[defaultType].m_setting;
+        if (name.isEmpty())
+            name = QString("Type %1").arg(defaultType);
+        LOG(VB_GENERAL, LOG_INFO,
+            QString("Grabber '%1' is not configured. Do you need to set PYTHONPATH?").arg(name));
+    }
+    return grabber;
 }
 
 MetaGrabberScript MetaGrabberScript::GetType(const QString &type)