Failure output does not follow standard `file:line` path format

When a test fails output is reported contains a file path, and on a new line the line number where the assertion fired.

This does not follow the standard form for reporting a location in a code base, and if we compare to the standard go test framework we would expect the output as file-path:line-number. This allows the path to be clickable in most IDEs and will take you directly to the problem line at hand.

Goconvey should also report file-paths like this.