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

Documented both of them

doc/function/assert.txt

@@ -16,3 +16,7 @@ Note: @assert@ is a special built-in keyword, so that the error message can incl
 --Examples--
 > assert(1 + 1 == 2)   # nothing happens
 > assert(1 * 1 == 2)   # An error message is shown
+
+--See also--
+| [[fun:warning]]		Output a warning message.
+| [[fun:error]]		Output an error message.

doc/function/error.txt

+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

@@ -109,3 +109,5 @@ 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/trace.txt

@@ -10,4 +10,8 @@ Output a message for debugging purposes, for example to investigate what is goin
 | @input@	[[type:string]]		Message
 
 --Examples--
-> trace("message") == "message"   # the message is reported in a message box
+> trace("message") == "message"   # the message is shown in the console panel
+
+--See also--
+| [[fun:warning]]		Output a warning message.
+| [[fun:error]]		Output an error message.

doc/function/warning.txt

+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
@@ -709,6 +718,8 @@ 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);