Cleanup -includeconf error message

Remove the erroneous trailing newline '\n'. Also, print only the first
value to remove needless redundancy in the error message.

src/util/system.cpp

@@ -366,14 +366,12 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
     }
 
     // we do not allow -includeconf from command line
-    bool success = true;
     if (auto* includes = util::FindKey(m_settings.command_line_options, "includeconf")) {
-        for (const auto& include : util::SettingsSpan(*includes)) {
-            error += "-includeconf cannot be used from commandline; -includeconf=" + include.write() + "\n";
-            success = false;
-        }
+        const auto& include{*util::SettingsSpan(*includes).begin()}; // pick first value as example
+        error = "-includeconf cannot be used from commandline; -includeconf=" + include.write();
+        return false;
     }
-    return success;
+    return true;
 }
 
 std::optional<unsigned int> ArgsManager::GetArgFlags(const std::string& name) const

test/functional/feature_includeconf.py

@@ -47,7 +47,7 @@ class IncludeConfTest(BitcoinTestFramework):
             expected_msg='Error: Error parsing command line arguments: -includeconf cannot be used from commandline; -includeconf=true',
         )
         self.nodes[0].assert_start_raises_init_error(
-            extra_args=['-includeconf=relative2.conf'],
+            extra_args=['-includeconf=relative2.conf', '-includeconf=no_warn.conf'],
             expected_msg='Error: Error parsing command line arguments: -includeconf cannot be used from commandline; -includeconf="relative2.conf"',
         )