Web: Clarify that `OS.get_unique_id` is not supported

Remove the base error message in `OS`, we no longer really error out this
way for not implemented methods. Instead, each platform should override them
to provide the context they want.

Fixes #82439.

(cherry picked from commit 0a10f09ce4321695940a626eef9c64b36f599193)

core/os/os.cpp

@@ -515,7 +515,7 @@ void OS::swap_buffers() {
 }
 
 String OS::get_unique_id() const {
-	ERR_FAIL_V("");
+	return "";
 }
 
 int OS::get_processor_count() const {

doc/classes/OS.xml

@@ -550,7 +550,7 @@
 			<description>
 				Returns a string that is unique to the device.
 				[b]Note:[/b] This string may change without notice if the user reinstalls/upgrades their operating system or changes their hardware. This means it should generally not be used to encrypt persistent data as the data saved before an unexpected ID change would become inaccessible. The returned string may also be falsified using external programs, so do not rely on the string returned by [method get_unique_id] for security purposes.
-				[b]Note:[/b] Returns an empty string on HTML5, as this method isn't implemented on this platform yet.
+				[b]Note:[/b] Returns an empty string and prints an error on HTML5, as this method cannot be implemented on this platform.
 			</description>
 		</method>
 		<method name="get_unix_time" qualifiers="const">

platform/javascript/os_javascript.cpp

@@ -1005,6 +1005,10 @@ int OS_JavaScript::get_processor_count() const {
 	return godot_js_os_hw_concurrency_get();
 }
 
+String OS_JavaScript::get_unique_id() const {
+	ERR_FAIL_V_MSG("", "OS::get_unique_id() is not available on the HTML5 platform.");
+}
+
 bool OS_JavaScript::_check_internal_feature_support(const String &p_feature) {
 	if (p_feature == "HTML5" || p_feature == "web") {
 		return true;

platform/javascript/os_javascript.h

@@ -194,6 +194,7 @@ public:
 	virtual int get_process_id() const;
 	bool is_process_running(const ProcessID &p_pid) const;
 	int get_processor_count() const;
+	String get_unique_id() const;
 
 	virtual void alert(const String &p_alert, const String &p_title = "ALERT!");
 	virtual void set_window_title(const String &p_title);