Improve test failure message

On os x if jsonschema is not installed the error message was cryptic.

hydra-node/test/Hydra/JSONSchema.hs

@@ -22,10 +22,12 @@ import System.Exit (ExitCode (..))
 import System.FilePath (normalise, takeBaseName, takeExtension, (<.>), (</>))
 import System.Process (readProcessWithExitCode)
 import Test.Hspec (pendingWith)
-import Test.Hydra.Prelude (createSystemTempDirectory)
+import Test.Hydra.Prelude (createSystemTempDirectory, failure)
 import Test.QuickCheck (Property, conjoin, counterexample, forAllBlind, forAllShrink, vectorOf, withMaxSuccess)
 import Test.QuickCheck.Monadic (assert, monadicIO, monitor, run)
 import qualified Prelude
+import System.IO.Error (IOError, ioeGetErrorType)
+import GHC.IO.Exception (IOErrorType(OtherError))
 
 -- | Generate arbitrary serializable (JSON) value, and check their validity
 -- against a known JSON schema.
@@ -158,12 +160,17 @@ ensureSystemRequirements ::
   IO ()
 ensureSystemRequirements = do
   getToolVersion >>= \case
-    Just "3.2.0" -> pure ()
-    _ -> pendingWith "This test requires the python library 'jsonschema==3.2.0' to be in scope."
+    Right "3.2.0" -> pure ()
+    Right _ -> failure "This test requires the python library 'jsonschema==3.2.0' to be in scope."
+    Left errorMsg -> failure errorMsg
  where
   -- Returns 'Nothing' when not available and 'Just <version number>' otherwise.
   getToolVersion ::
-    IO (Maybe String)
+    IO (Either String String)
   getToolVersion = do
-    (exitCode, out, _) <- readProcessWithExitCode "jsonschema" ["--version"] mempty
-    pure (dropWhileEnd isSpace out <$ guard (exitCode == ExitSuccess))
+    try (readProcessWithExitCode "jsonschema" ["--version"] mempty) >>= \case
+      Right (exitCode, out, _) -> pure (dropWhileEnd isSpace out <$ guard (exitCode == ExitSuccess))
+      Left (err :: IOError)
+        | ioeGetErrorType err == OtherError ->
+            pure (Left $ "Check jsonschema is installed and in $PATH")
+      Left err -> pure (Left $ show err)