Shell: Add runtime errors and implement break/continue

Such errors are raised when SyntaxError nodes are executed, and are also
used for internal control flow.
The 'break' and 'continue' commands are currently only allowed inside
for loops, and outside function bodies.

This also adds a 'loop' keyword for infinite loops.

Shell/Shell.h

@@ -201,6 +201,31 @@ public:
         ReadLine,
     };
 
+    enum class ShellError {
+        None,
+        InternalControlFlowBreak,
+        InternalControlFlowContinue,
+        EvaluatedSyntaxError,
+        NonExhaustiveMatchRules,
+        InvalidGlobError,
+    };
+
+    void raise_error(ShellError kind, String description)
+    {
+        m_error = kind;
+        m_error_description = move(description);
+    }
+    bool has_error(ShellError err) const { return m_error == err; }
+    const String& error_description() const { return m_error_description; }
+    ShellError take_error()
+    {
+        auto err = m_error;
+        m_error = ShellError::None;
+        m_error_description = {};
+        return err;
+    }
+    void possibly_print_error() const;
+
 #define __ENUMERATE_SHELL_OPTION(name, default_, description) \
     bool name { default_ };
 
@@ -263,6 +288,9 @@ private:
     bool m_is_subshell { false };
     bool m_should_reinstall_signal_handlers { true };
 
+    ShellError m_error { ShellError::None };
+    String m_error_description;
+
     bool m_should_format_live { false };
 
     RefPtr<Line::Editor> m_editor;