Fix zcash-cli crash when printing help message

When a `zcash-cli` command fails, it attempts to print the help message for the command. However,
making the `help` call can also fail, and there was a bug in this check, so that we tried to display
the help message when the `help` call failed, and tried to display the error when the `help` call
succeeded – both leading to an assertion failure.

This also makes some minor changes to the output formatting.

Fixes #6561

src/bitcoin-cli.cpp

@@ -212,12 +212,12 @@ UniValue ConvertValues(const std::string &strMethod, const std::vector<std::stri
                             auto helpMsg = CallRPC("help", ConvertValues("help", {strMethod}));
                             return "\n\n"
                                 + (helpMsg.has_value()
-                                   ? strprintf(
+                                   ? "Usage: " + helpMsg.value().get_str()
+                                   : strprintf(
                                            "An error occurred while attempting to retrieve the "
                                            "help text for %s: %s",
                                            strMethod,
-                                           helpMsg.error().get_str())
-                                   : helpMsg->get_str());
+                                           helpMsg.error().get_str()));
                         }
                     }));
         })