Make error description available as "%val{error}" during catch blocks

Fixes #2761

doc/pages/commands.asciidoc

@@ -278,7 +278,9 @@ but not really useful in that context.
     done on error, the catch part can be omitted. If an error is raised
     in the *on_error_commands*, that error is propagated, except if
     another *catch* and *on_error_commands* parameter follows, in which
-    case those commands get executed, and so-on.
+    case those commands get executed, and so-on. During error commands,
+    the description of the last raiser error is available as `$kak_error`
+    in the shell, or `%val{error}` in commands.
 
 *nop*::
     does nothing, but arguments will be evaluated (e.g. shell expansion)

src/commands.cc

@@ -2080,17 +2080,26 @@ const CommandDesc try_catch_cmd = {
         }
 
         CommandManager& command_manager = CommandManager::instance();
+        Optional<ShellContext> shell_context_with_error;
         for (size_t i = 0; i < parser.positional_count(); i += 2)
         {
             if (i == 0 or i < parser.positional_count() - 1)
             {
-                try {
-                    command_manager.execute(parser[i], context, shell_context);
+                try
+                {
+                    command_manager.execute(parser[i], context,
+                                            shell_context_with_error.value_or(shell_context));
                     return;
-                } catch (runtime_error&) {}
+                }
+                catch (const runtime_error& error)
+                {
+                    shell_context_with_error.emplace(shell_context);
+                    shell_context_with_error->env_vars[StringView{"error"}] = error.what().str();
+                }
             }
             else
-                command_manager.execute(parser[i], context, shell_context);
+                command_manager.execute(parser[i], context,
+                                        shell_context_with_error.value_or(shell_context));
         }
     }
 };

test/compose/catch-error-desc/cmd

@@ -0,0 +1 @@
+:error<ret>

test/compose/catch-error-desc/in

@@ -0,0 +1 @@
+

test/compose/catch-error-desc/out

@@ -0,0 +1 @@
+no such command: 'non-existing-command'

test/compose/catch-error-desc/rc

@@ -0,0 +1 @@
+define-command error %{ try %{ non-existing-command } catch %{ exec i "%val{error}" <esc> } }