v0.5.0: Better flattening and nil safety

• Does not panic when passed a nil error (#20)
• Resolves empty error messages in certain cases (#7, #16)
• Yield a more logical error chain for errors wrapped with fmt.Errorf and pkg/errors (#24)
• Don't duplicate messages even when the .Error() implementation includes a duplicate of the nested error (#24)
• Simplify the Flatten output to just a slice of steps in the error chain, no more Root as index [0] already contains the root (#11)