When running javac, print out the number of errors and warnings.

Summary:
When trying to fix a java_library with a lot of errors, it is helpful
to see the error count as you go along so you can tell if you are making
progress or not.

Test Plan: Built a failing java_library and verify that the message appears.

src/com/facebook/buck/java/JavacInMemoryStep.java

diff --git a/src/com/facebook/buck/java/JavacInMemoryStep.java b/src/com/facebook/buck/java/JavacInMemoryStep.java
index 43a1c40..cab2500 100644
--- a/src/com/facebook/buck/java/JavacInMemoryStep.java
+++ b/src/com/facebook/buck/java/JavacInMemoryStep.java
@@ -321,9 +321,22 @@
       return 0;
     } else {
       if (context.getVerbosity().shouldPrintStandardInformation()) {
+        int numErrors = 0;
+        int numWarnings = 0;
         for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
+          Diagnostic.Kind kind = diagnostic.getKind();
+          if (kind == Diagnostic.Kind.ERROR) {
+            ++numErrors;
+          } else if (kind == Diagnostic.Kind.WARNING || kind == Diagnostic.Kind.MANDATORY_WARNING) {
+            ++numWarnings;
+          }
+
           context.getStdErr().println(diagnostic);
         }
+
+        if (numErrors > 0 || numWarnings > 0) {
+          context.getStdErr().printf("Errors: %d. Warnings: %d.\n", numErrors, numWarnings);
+        }
       }
       return 1;
     }