COMPMID-417 Allow the tests to run even when assets are not present

Change-Id: Ief165b1d583a70cbe35aae93f05ddfe962196323
Reviewed-on: http://mpd-gerrit.cambridge.arm.com/89503
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
Tested-by: Kaizen <jeremy.johnson+kaizengerrit@arm.com>
diff --git a/tests/AssetsLibrary.cpp b/tests/AssetsLibrary.cpp
index 529a2a2..7e380fa 100644
--- a/tests/AssetsLibrary.cpp
+++ b/tests/AssetsLibrary.cpp
@@ -153,7 +153,7 @@
 
     if(!file.good())
     {
-        throw std::runtime_error("Could not load PPM image: " + path);
+        throw framework::FileNotFound("Could not load PPM image: " + path);
     }
 
     unsigned int width  = 0;