Stop Coverage keyword should tell if it could not find file defined in config_file argument in Start Coverage keyword

[aaltat]

Describe the bug
If I have test that looks like this

*** Settings ***
Library    Browser

*** Test Cases ***
Example
    New Page    http://www.google.com
    Start Coverage    config_file=config.js
    Stop Coverage

Then Stop Coverage coverage keyword will say that all is fine although it could not locate the config.js file. Correct way of using the config_file argument would have been config_file=${CURDIR}/config.js.

We should enhance the discovery of the config file, if full path is not defined. Also if file is not still found, Start and Stop Coverage keywords should enhance the log message that could not locate the config file. Now we log that all is OK, which is misleading because configuration file was not found although report was created.