Add errors.Cause to find inital cause of error.

[jrick]

This will simplify situations where calling code must unwrap the error chain for
other errors and error types.

[jrick]

This will require some extra changes or additional documentation to cover this corner case:

errors.Cause(errors.E("error")) // type is errors.errorString from the stdlib

This is due to how E uses stdlib errors.New is used to create errors from strings.