Expose the 'warning' function, and added a similar 'error' function.

Add documentation.

doc/function/error.txt

@@ -0,0 +1,21 @@
+Function: error
+
+DOC_MSE_VERSION: since 2.0.1
+
+--Usage--
+> error(some_string)
+
+Output an error message.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:string]]		Message
+| @condition@	[[type:boolean]]	(optional) Only show the message if this condition holds.
+
+--Examples--
+> error("message")   # the message is shown in the console panel
+> error("message", condition: false)  # shows no message
+
+--See also--
+| [[fun:assert]]		Check that the condition is @true@. Shows a warning message if it is not.
+| [[fun:warning]]		Output a warning message.

doc/function/index.txt

@@ -110,3 +110,5 @@ These functions are built into the program, other [[type:function]]s can be defi
 ! Other functions		<<<
 | [[fun:trace]]			Output a message for debugging purposes.
 | [[fun:assert]]		Check a condition for debugging purposes.
+| [[fun:warning]]		Output a warning message.
+| [[fun:error]]			Output an error message.

doc/function/warning.txt

@@ -0,0 +1,21 @@
+Function: warning
+
+DOC_MSE_VERSION: since 2.0.1
+
+--Usage--
+> warning(some_string)
+
+Output a warning message.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:string]]		Message
+| @condition@	[[type:boolean]]	(optional) Only show the message if this condition holds.
+
+--Examples--
+> warning("message")"   # the message is shown in the console panel
+> warning("message", condition: false)  # shows no message
+
+--See also--
+| [[fun:assert]]		Check that the condition is @true@. Shows a warning message if it is not.
+| [[fun:error]]		Output an error message.

src/script/functions/basic.cpp

@@ -56,6 +56,15 @@ SCRIPT_FUNCTION(warning_if_neq) {
   return script_nil;
 }
 
+SCRIPT_FUNCTION(error) {
+  SCRIPT_PARAM_C(String, input);
+  SCRIPT_PARAM_DEFAULT_C(bool, condition, true);
+  if (condition) {
+    queue_message(MESSAGE_ERROR, input);
+  }
+  return script_nil;
+}
+
 // ----------------------------------------------------------------------------- : Conversion
 
 /// Format the input variable based on a printf like style specification
@@ -714,6 +723,8 @@ SCRIPT_FUNCTION(rule) {
 void init_script_basic_functions(Context& ctx) {
   // debugging
   ctx.setVariable(_("trace"),                script_trace);
+  ctx.setVariable(_("warning"),              script_warning);
+  ctx.setVariable(_("error"),                script_error);
   // conversion
   ctx.setVariable(_("to_string"),            script_to_string);
   ctx.setVariable(_("to_int"),               script_to_int);