Cycles: Report human-readable string of compilation error code

It is possible that compilation will fail without giving anything in the
log buffer. For this cases giving a tip about error code will be really
handy.

Patch by @Ilia, thanks!

intern/cycles/device/device_opencl.cpp

@@ -875,6 +875,7 @@ public:
 
 		if(ciErr != CL_SUCCESS) {
 			opencl_error("OpenCL build failed: errors in console");
+			fprintf(stderr, "Build error: %s\n", clewErrorString(ciErr));
 			return false;
 		}