New error-system for Etcd with docs

Documentation/errorcode.md

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+Error Code
+======
+
+This document describes the error code in **Etcd** project.
+
+It's categorized into four groups:
+
+- Command Related Error
+- Post Form Related Error
+- Raft Related Error
+- Etcd Related Error
+
+Error code corresponding strerror
+------
+
+    const (
+        EcodeKeyNotFound    = 100
+        EcodeTestFailed     = 101
+        EcodeNotFile        = 102
+        EcodeNoMoreMachine  = 103
+        EcodeNotDir         = 104
+        EcodeNodeExist      = 105
+        EcodeKeyIsPreserved = 106
+
+        EcodeValueRequired     = 200
+        EcodePrevValueRequired = 201
+        EcodeTTLNaN            = 202
+        EcodeIndexNaN          = 203
+
+        EcodeRaftInternal = 300
+        EcodeLeaderElect  = 301
+
+        EcodeWatcherCleared = 400
+        EcodeEventIndexCleared = 401
+    )
+
+    // command related errors
+    errors[100] = "Key Not Found"
+    errors[101] = "Test Failed" //test and set
+    errors[102] = "Not A File"
+    errors[103] = "Reached the max number of machines in the cluster"
+    errors[104] = "Not A Directory"
+    errors[105] = "Already exists" // create
+    errors[106] = "The prefix of given key is a keyword in etcd"
+
+    // Post form related errors
+    errors[200] = "Value is Required in POST form"
+    errors[201] = "PrevValue is Required in POST form"
+    errors[202] = "The given TTL in POST form is not a number"
+    errors[203] = "The given index in POST form is not a number"
+
+    // raft related errors
+    errors[300] = "Raft Internal Error"
+    errors[301] = "During Leader Election"
+
+    // etcd related errors
+    errors[400] = "watcher is cleared due to etcd recovery"
+    errors[401] = "The event in requested index is outdated and cleared"