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/test/app/Regression_test.cpp

@@ -149,7 +149,7 @@ struct Regression_test : public beast::unit_test::suite
             secp256r1Sig->setFieldVL(sfSigningPubKey, *pubKeyBlob);
             jt.stx.reset(secp256r1Sig.release());
 
-            env(jt, ter(temINVALID));
+            env(jt, ter(telENV_RPC_FAILED));
         };
 
         Account const alice{"alice", KeyType::secp256k1};