Add a test making sure pattern match failures on GhcModT work the way I expect

ghc-mod.cabal

@@ -169,6 +169,7 @@ Test-Suite spec
                         LangSpec
                         LintSpec
                         ListSpec
+                        MonadSpec
                         GhcPkgSpec
                         TestUtils
   Build-Depends:        base >= 4.0 && < 5

test/MonadSpec.hs

@@ -0,0 +1,19 @@
+module MonadSpec where
+
+import Test.Hspec
+import Control.Monad.Error.Class
+import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.Monad
+import Language.Haskell.GhcMod.Find
+
+spec :: Spec
+spec = do
+    describe "When using GhcModT in a do block" $
+        it "a pattern match failure causes a call to `fail` on ErrorT in the monad stack" $ do
+             (a, w)
+                 <- runGhcModT defaultOptions $
+                       do
+                         Just a <- return Nothing
+                         return "hello"
+                     `catchError` (const $ fail "oh noes")
+             a `shouldBe` (Left $ GMEString "oh noes")