Error code framework

As one of the feedbacks from the RetroArch UI Improvements
series put together by Nic Watt, the ways RetroArch (or any
other frontend) could indicate errors about the cores, is quite
limited. The Libretro API functions provide little feedback
opportunity for the cores to indicate problems.

By introducing a new environment call, the possibility will be
available across the board. This change is a wireframe and may be
extended / adopted as needed.

Author: zoltanvb

libretro-common/include/libretro.h

@@ -711,6 +711,20 @@ enum retro_mod
    RETROKMOD_DUMMY = INT_MAX /* Ensure sizeof(enum) == sizeof(int) */
 };
 
+/* Error codes that can be reported by the core. */
+enum retro_error
+{
+   RETROE_NONE           = 0x00000000,
+   RETROE_MASK_CORE      = 0x0000FFFF, /* Mask value for core/frontend split */
+
+   RETROE_UNSUPPORTED_CONTENT_FORMAT       = 0x01000000,
+   RETROE_MISSING_BIOS                     = 0x02000000,
+   RETROE_HARDWARE_RENDERING_NOT_AVAILABLE = 0x03000000,
+
+   RETROE_MASK_FRONTEND = 0x7FFF0000, /* Mask value for core/frontend split - limited to 31 bit due to C89 */
+   RETROE_DUMMY         = INT_MAX     /* Ensure sizeof(enum) == sizeof(int) */
+};
+
 /**
  * @defgroup RETRO_ENVIRONMENT Environment Callbacks
  * @{
@@ -2574,6 +2588,17 @@ enum retro_mod
  */
 #define RETRO_ENVIRONMENT_GET_FILE_BROWSER_START_DIRECTORY 80
 
+/**
+ * Informs the frontend of an error occurrence during the current operation.
+ *
+ * @param[in] data <tt>const uint32_t*</tt>.
+ * Valid values of the upper 16 bit are defined as retro_error.
+ * The lower 16 bits are reserved for the core.
+ * @return \c true if the environment call is available.
+ * @see retro_error
+ */
+#define RETRO_ENVIRONMENT_SET_ERROR_CODE 81
+
 /**@}*/
 
 /**