ui: Initial better user error reporting (#1503)

This update the "No keys" dialog and block starting NSP/XCI/NCA without firmware.

Also propose to the user if they want to install firmware if they start an untrimmed XCI and remove KEYS.md as it was completely outdated.

PS: Also fix a bug with "&" in URL with OpenUrl on Windows.

Ryujinx/Ui/Diagnostic/UserError.cs

@@ -0,0 +1,39 @@
+namespace Ryujinx.Ui.Diagnostic
+{
+    /// <summary>
+    /// Represent a common error that could be reported to the user by the emulator.
+    /// </summary>
+    public enum UserError
+    {
+        /// <summary>
+        /// No error to report.
+        /// </summary>
+        Success = 0x0,
+
+        /// <summary>
+        /// No keys are present.
+        /// </summary>
+        NoKeys = 0x1,
+
+        /// <summary>
+        /// No firmware is installed.
+        /// </summary>
+        NoFirmware = 0x2,
+
+        /// <summary>
+        /// Firmware parsing failed.
+        /// </summary>
+        /// <remarks>Most likely related to keys.</remarks>
+        FirmwareParsingFailed = 0x3,
+
+        /// <summary>
+        /// No application was found at the given path.
+        /// </summary>
+        ApplicationNotFound = 0x4,
+
+        /// <summary>
+        /// An unknown error.
+        /// </summary>
+        Unknown = 0xDEAD
+    }
+}