Avoid running into an IOException and logging it.

When we recently switched from System.err printing to slf4j logging,
we started logging an IOException that we shouldn't be running into
and that we simply ignored before.  This exception gets thrown when
DescriptorReaderImpl attempts to read a parse history file that
doesn't exist (yet).  We should simply check whether that files exists
before attempting to read it.

Fixes #20320.

CHANGELOG.md

+# Changes in version 1.?.? - 2016-1?-??
+
+ * Minor changes
+   - Avoid running into an IOException and logging a warning for it.
+
+
 # Changes in version 1.4.0 - 2016-08-31
 
  * Major changes

src/main/java/org/torproject/descriptor/impl/DescriptorReaderImpl.java

@@ -200,7 +200,7 @@ public class DescriptorReaderImpl implements DescriptorReader {
     }
 
     private void readOldHistory() {
-      if (this.historyFile == null) {
+      if (this.historyFile == null || !this.historyFile.exists()) {
         return;
       }
       try {