Log parsing errors instead of failing (#125)

1. Split the class hierarchy construction front-end parsing error
checking into two.
2. Log parsing errors instead of throwing an exception.

com.ibm.wala.cast.python/source/com/ibm/wala/cast/python/client/PythonAnalysisEngine.java

@@ -58,14 +58,14 @@
 import com.ibm.wala.types.TypeReference;
 import com.ibm.wala.util.CancelException;
 import com.ibm.wala.util.WalaException;
-import com.ibm.wala.util.WalaRuntimeException;
 import com.ibm.wala.util.collections.HashMapFactory;
 import com.ibm.wala.util.collections.HashSetFactory;
 import java.io.IOException;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
+import java.util.logging.Level;
 import java.util.logging.Logger;
 
 public abstract class PythonAnalysisEngine<T>
@@ -135,17 +135,26 @@ public void buildAnalysisScope() throws IOException {
 
   @Override
   public IClassHierarchy buildClassHierarchy() {
+    IClassHierarchy cha = null;
+
     try {
-      IClassHierarchy cha = SeqClassHierarchyFactory.make(scope, loader);
-      Util.checkForFrontEndErrors(cha);
-      setClassHierarchy(cha);
-      return cha;
+      cha = SeqClassHierarchyFactory.make(scope, loader);
     } catch (ClassHierarchyException e) {
       assert false : e;
       return null;
+    }
+
+    try {
+      Util.checkForFrontEndErrors(cha);
     } catch (WalaException e) {
-      throw new WalaRuntimeException(e.getMessage(), e);
+      logger.log(
+          Level.WARNING,
+          e,
+          () -> "Encountered WALA exception, most likely from front-end parsing errors.");
     }
+
+    setClassHierarchy(cha);
+    return cha;
   }
 
   protected void addSummaryBypassLogic(AnalysisOptions options, String summary) {