Better error message when setting values on directories

Without this commit etcd returns the following error message when
setting values on directories:

    {
      "errorCode":102,
      "message":"Not a file",
      "cause":"/postgres",
      "index":2
    }

While the above error message is accurate it's not very descriptive.
This commit adds a new error code/message which better describes why the
write operation failed. etcd now returns the following:

    {
      "errorCode":109,
      "message":"Cannot set value on directory",
      "cause":"/postgres",
      "index":2
    }

Documentation/errorcode.md

@@ -21,6 +21,7 @@ Error code corresponding strerror
         EcodeNotDir         = 104
         EcodeNodeExist      = 105
         EcodeKeyIsPreserved = 106
+        EcodeNoValueOnDir   = 109
 
         EcodeValueRequired     = 200
         EcodePrevValueRequired = 201
@@ -42,6 +43,7 @@ Error code corresponding strerror
     errors[104] = "Not A Directory"
     errors[105] = "Already exists" // create
     errors[106] = "The prefix of given key is a keyword in etcd"
+    errors[109] = "Cannot set value on directory"
 
     // Post form related errors
     errors[200] = "Value is Required in POST form"