test: Env unit test RPC errors return a unique result: (#4877)

* telENV_RPC_FAILED is a new code, reserved exclusively
  for unit tests when RPC fails. This will
  make those types of errors distinct and easier to test
  for when expected and/or diagnose when not.
* Output RPC command result when result is not expected.

src/ripple/protocol/TER.h

@@ -64,7 +64,8 @@ enum TELcodes : TERUnderlyingType {
     telCAN_NOT_QUEUE_FULL,
     telWRONG_NETWORK,
     telREQUIRES_NETWORK_ID,
-    telNETWORK_ID_MAKES_TX_NON_CANONICAL
+    telNETWORK_ID_MAKES_TX_NON_CANONICAL,
+    telENV_RPC_FAILED
 };
 
 //------------------------------------------------------------------------------

src/ripple/protocol/impl/TER.cpp

@@ -154,6 +154,7 @@ transResults()
         MAKE_ERROR(telWRONG_NETWORK,          "Transaction specifies a Network ID that differs from that of the local node."),
         MAKE_ERROR(telREQUIRES_NETWORK_ID,    "Transactions submitted to this node/network must include a correct NetworkID field."),
         MAKE_ERROR(telNETWORK_ID_MAKES_TX_NON_CANONICAL, "Transactions submitted to this node/network must NOT include a NetworkID field."),
+        MAKE_ERROR(telENV_RPC_FAILED,         "Unit test RPC failure."),
 
         MAKE_ERROR(temMALFORMED,                 "Malformed transaction."),
         MAKE_ERROR(temBAD_AMM_TOKENS,            "Malformed: Invalid LPTokens."),