Catch readFile exceptions on the repl

[thumphries]

Exceptions thrown by readDeclaration make it impossible to write properties inside GHCi. This PR just catches them.

I think this sufficiently addresses #105, the @chris-martin example from that issue now works.

λ> import Hedgehog
λ> import qualified Hedgehog.Gen as Gen
λ> import qualified Hedgehog.Range as Range
λ> check $ property $ forAll (Gen.integral $ Range.exponential 1 1024) >>= \x -> x + 1 === x
  ✗ <interactive> failed after 1 test.
  
    forAll0 =
      1
    
    This failure can be reproduced by running:
    > recheck (Size 0) (Seed 7634626840477011566 (-6948853007075827919)) <property>
  
False

[thumphries]

I dunno if this is actually clearer than MaybeT

[jacobstanley]