Rework fail command not to display command call stack

`fail` triggers "expected" errors, and hence should just display
the provided message.

src/command_manager.cc

@@ -425,10 +425,15 @@ void CommandManager::execute_single_command(CommandParameters params,
                                           command_it->value.param_desc);
         command_it->value.func(parameter_parser, context, shell_context);
     }
+    catch (failure& error)
+    {
+        throw;
+    }
     catch (runtime_error& error)
     {
-        throw runtime_error(format("{}:{}: '{}' {}", pos.line+1, pos.column+1,
+        error.set_what(format("{}:{}: '{}' {}", pos.line+1, pos.column+1,
-                                   params[0], error.what()));
+                              params[0], error.what()));
+        throw;
     }
 }
 

src/commands.cc

@@ -2140,7 +2140,7 @@ const CommandDesc fail_cmd = {
     CommandCompleter{},
     [](const ParametersParser& parser, Context&, const ShellContext&)
     {
-        throw runtime_error(fix_atom_text(join(parser, ' ', false)));
+        throw failure{fix_atom_text(join(parser, " "))};
     }
 };
 

src/exception.hh

@@ -18,11 +18,17 @@ struct runtime_error : exception
         : m_what(std::move(what)) {}
 
     StringView what() const override { return m_what; }
+    void set_what(String what) { m_what = std::move(what); }
 
 private:
     String m_what;
 };
 
+struct failure : runtime_error
+{
+    using runtime_error::runtime_error;
+};
+
 struct logic_error : exception
 {
 };