Add Precondition Failed code (412) for users of the Compute API.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133154438

Author: zoltak-g

google-http-client/src/main/java/com/google/api/client/http/HttpStatusCodes.java

@@ -68,6 +68,9 @@ public class HttpStatusCodes {
 
   /** Status code for a request that could not be completed due to a resource conflict. */
   public static final int STATUS_CODE_CONFLICT = 409;
+  
+  /** Status code for a request for which one of the conditions it was made under has failed. */
+  public static final int STATUS_CODE_PRECONDITION_FAILED = 412;
 
   /** Status code for an internal server error. */
   public static final int STATUS_CODE_SERVER_ERROR = 500;