(svn r24821) -Add: When using a non-release version of OpenTTD and the basegraphics are missing some sprites, also suggest to use a non-release version of the basegraphics.

src/lang/english.txt

@@ -2716,6 +2716,7 @@ STR_NEWGRF_ERROR_CORRUPT_SPRITE                                 :{YELLOW}{RAW_ST
 STR_NEWGRF_ERROR_MULTIPLE_ACTION_8                              :Contains multiple Action 8 entries (sprite {3:NUM})
 STR_NEWGRF_ERROR_READ_BOUNDS                                    :Read past end of pseudo-sprite (sprite {3:NUM})
 STR_NEWGRF_ERROR_MISSING_SPRITES                                :{WHITE}The currently used base graphics set is missing a number of sprites.{}Please update the base graphics set
+STR_NEWGRF_ERROR_MISSING_SPRITES_UNSTABLE                       :{WHITE}The currently used base graphics set is missing a number of sprites.{}Please update the base graphics set.{}Since you are playing a {YELLOW}development snapshot of OpenTTD{WHITE}, you might also need a {YELLOW}development snapshot of the base graphics{WHITE}
 STR_NEWGRF_ERROR_GRM_FAILED                                     :Requested GRF resources not available (sprite {3:NUM})
 STR_NEWGRF_ERROR_FORCEFULLY_DISABLED                            :{1:RAW_STRING} was disabled by {2:RAW_STRING}
 STR_NEWGRF_ERROR_INVALID_SPRITE_LAYOUT                          :Invalid/unknown sprite layout format (sprite {3:NUM})

src/newgrf.cpp

@@ -5595,7 +5595,7 @@ void CheckForMissingSprites()
 	}
 
 	if (missing) {
-		ShowErrorMessage(STR_NEWGRF_ERROR_MISSING_SPRITES, INVALID_STRING_ID, WL_CRITICAL);
+		ShowErrorMessage(IsReleasedVersion() ? STR_NEWGRF_ERROR_MISSING_SPRITES : STR_NEWGRF_ERROR_MISSING_SPRITES_UNSTABLE, INVALID_STRING_ID, WL_CRITICAL);
 	}
 }