Fix HNAS driver confusing error message

The error message shown when the parser finds a parser error
says, 'file not found' which causes confusion on the user when
he/she needs to debug the real cause o the problem. This patch fix
this by testing first if the file exist and then throwing a
proper error message.

Closes-Bug: #1402775
Change-Id: I91c7a24d5da37735787e8fc0da544c8ba8204884
Merge "Fix HNAS driver confusing error message"