Improve CopyStep's error message.

Summary:
I saw this while developing, and I was not sure what was attempted
to be copied.

Test Plan: Sandcastle builds.

src/com/facebook/buck/step/fs/CopyStep.java

diff --git a/src/com/facebook/buck/step/fs/CopyStep.java b/src/com/facebook/buck/step/fs/CopyStep.java
index c1305f9..34f5c12 100644
--- a/src/com/facebook/buck/step/fs/CopyStep.java
+++ b/src/com/facebook/buck/step/fs/CopyStep.java
@@ -48,7 +48,7 @@
   }
 
   @Override
-  public String getDescription (ExecutionContext context) {
+  public String getDescription(ExecutionContext context) {
     ImmutableList.Builder<String> args = ImmutableList.builder();
     args.add("cp");
     if (shouldRecurse) {
@@ -83,7 +83,7 @@
       }
       return 0;
     } catch (IOException e) {
-      e.printStackTrace(context.getStdErr());
+      context.logError(e, "Failed when trying to copy: %s", getDescription(context));
       return 1;
     }
   }