Add a fail command to explicitely raise an error

README.asciidoc

@@ -725,6 +725,7 @@ command `q!` has to be used).
      `:%sh{ echo echo tchou }` will echo tchou in Kakoune, whereas
      `:nop %sh{ echo echo tchou }` will not, but both will execute the
      shell command.
+ * `fail <text>`: raise an error, uses <text> as its description
 
 Multiple commands
 ~~~~~~~~~~~~~~~~~

doc/manpages/commands.asciidoc

@@ -96,6 +96,9 @@ command *q!* has to be used). Aliases are mentionned below each commands.
 *nop*::
 	does nothing, but arguments will be evaluated (e.g. shell expansion)
 
+*fail* <text>::
+	raise an error, uses <text> as its description
+
 *declare-option* [-hidden] <type> <name> [<value>]::
 	*alias* decl +
 	declare a new option, the -hidden hides the option in completion

src/commands.cc

@@ -2105,6 +2105,20 @@ const CommandDesc rename_session_cmd = {
     }
 };
 
+const CommandDesc fail_cmd = {
+    "fail",
+    nullptr,
+    "fail [<message>]: raise an error with the given message",
+    ParameterDesc{},
+    CommandFlags::None,
+    CommandHelper{},
+    CommandCompleter{},
+    [](const ParametersParser& parser, Context&, const ShellContext&)
+    {
+        throw runtime_error(fix_atom_text(join(parser, ' ', false)));
+    }
+};
+
 }
 
 void register_commands()
@@ -2166,6 +2180,7 @@ void register_commands()
     register_command(select_cmd);
     register_command(change_directory_cmd);
     register_command(rename_session_cmd);
+    register_command(fail_cmd);
 }
 
 }