Same as the Mid() VBA builtin string function, but if there's an error just return the empty string.
The string to search
The start position of the substring
The number of characters to return, or -1 for the rest of the string
xf.Str.SafeMid("The King's counting house", 12) => counting house
xf.Str.SafeMid("The King's counting house", 12, 5) => count
xf.Str.SafeMid("The King's counting house", 35, 5) =>