Step failure output mangled

[sblundy]

Describe the bug

If a step fails because the step function returned an error, the output is mangled with fmt.SPrintf() error output.

For example, say the step is And 2 items posted. If the function returns fmt.Errorf("expected %d items, received %d", n, actual), then the output would be

gobdd.go:377: And %!(EXTRA string=2 items posted, *errors.errorString=expected 2 items posted, received 0)

Expected behavior

Prints out the step and the error clearly formatted. Something like:

gobdd.go:377: And 2 items posted: expected 2 items posted, received 0