[lldb] Improve error message for script commands when there's no interpreter

It was:
```
error: there is no embedded script interpreter in this mode.
```

1. What does "mode" mean?
2. It implies there might be an embedded script interpreter for some
   other "mode", whatever that would be.

So I'm simplifying it and noting the most common reason for this which
is that lldb wasn't built with a scripting lanugage enabled in the
first place.

There are other tips for dealing with this, but I'm not sure this message
is the best place for them.

Author: DavidSpickett

lldb/source/Plugins/ScriptInterpreter/None/ScriptInterpreterNone.cpp

@@ -26,17 +26,19 @@ ScriptInterpreterNone::ScriptInterpreterNone(Debugger &debugger)
 
 ScriptInterpreterNone::~ScriptInterpreterNone() = default;
 
+static const char *no_interpreter_err_msg =
+    "error: Embedded script interpreter unavailable. LLDB was built without "
+    "scripting language support.\n";
+
 bool ScriptInterpreterNone::ExecuteOneLine(llvm::StringRef command,
                                            CommandReturnObject *,
                                            const ExecuteScriptOptions &) {
-  m_debugger.GetErrorStream().PutCString(
-      "error: there is no embedded script interpreter in this mode.\n");
+  m_debugger.GetErrorStream().PutCString(no_interpreter_err_msg);
   return false;
 }
 
 void ScriptInterpreterNone::ExecuteInterpreterLoop() {
-  m_debugger.GetErrorStream().PutCString(
-      "error: there is no embedded script interpreter in this mode.\n");
+  m_debugger.GetErrorStream().PutCString(no_interpreter_err_msg);
 }
 
 void ScriptInterpreterNone::Initialize() {