rpc: Use type-safe exception to pass RPC help

src/rpc/server.cpp

@@ -97,11 +97,8 @@ std::string CRPCTable::help(const std::string& strCommand, const JSONRPCRequest&
             UniValue unused_result;
             if (setDone.insert(pcmd->unique_id).second)
                 pcmd->actor(jreq, unused_result, /*last_handler=*/true);
-        }
-        catch (const std::exception& e)
-        {
-            // Help text is returned in an exception
-            std::string strHelp = std::string(e.what());
+        } catch (const HelpResult& e) {
+            std::string strHelp{e.what()};
             if (strCommand == "")
             {
                 if (strHelp.find('\n') != std::string::npos)

src/rpc/util.cpp

@@ -645,7 +645,7 @@ UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request) const
      * the user is asking for help information, and throw help when appropriate.
      */
     if (request.mode == JSONRPCRequest::GET_HELP || !IsValidNumArgs(request.params.size())) {
-        throw std::runtime_error(ToString());
+        throw HelpResult{ToString()};
     }
     UniValue arg_mismatch{UniValue::VOBJ};
     for (size_t i{0}; i < m_args.size(); ++i) {

src/rpc/util.h

@@ -67,6 +67,10 @@ class FillableSigningProvider;
 class CScript;
 struct Sections;
 
+struct HelpResult : std::runtime_error {
+    explicit HelpResult(const std::string& msg) : std::runtime_error{msg} {}
+};
+
 /**
  * Gets all existing output types formatted for RPC help sections.
  *