(WIP, RFC) Distinguish variable-not-found-error from others

[igrep]

Problem

• Handling Maybe values in the Parser monad is troublesome.
• Even envMaybe mixes up variable-not-found-error with others, which makes the difference of Nothing and Left String obscure.

Solution

Change the error type of Parser a from just a String into a dedicated error ADT ParseError.

NOTE

• Fix https://github.com/dmjio/envy/issues/30 as a bonus!