***Python 3.6***

Define a function (not a script) called count_in_file() that takes two strings (a filename and a search string) and returns the number of times that the search string occurs in the file. If no such file exists, or if an OSError occurs while working with the file, the function should return -1.

You must use a for loop to read through the file line-by-line. That means that you cannot use .read(), .readline(), or .readlines().