Return a 404 instead of a 500 if a resource is unavailable.

Could be improved in the future to return different codes depending on its status (e.g. doesn't exist = 404, 102 for loading, 500 for error, etc), but 404 makes the most sense until that has been developed

src/main/java/org/qortal/api/resource/ArbitraryResource.java

@@ -641,7 +641,7 @@ public class ArbitraryResource {
 			return response;
 		} catch (Exception e) {
 			LOGGER.info(String.format("Unable to load %s %s: %s", service, name, e.getMessage()));
-			throw ApiExceptionFactory.INSTANCE.createCustomException(request, ApiError.REPOSITORY_ISSUE, e.getMessage());
+			throw ApiExceptionFactory.INSTANCE.createCustomException(request, ApiError.FILE_NOT_FOUND, e.getMessage());
 		}
 	}