Checker renamed: jc.IsNil -> jc.ErrorIsNil

[Tim Penhey]

Hi all,

After consideration and email conflab with Roger and John, I changed the
jc.IsNil checker to be more explicit.

Due to the expectations of errors being either valid or 'nil' the
ErrorIsNil check actually fails if the value is a nil value, but not
'nil', by that I mean a typed-nil.

The checker also expects that what-ever is passed through, if not nil,
supports the `error` interface.

This change has landed in master.

Tim