Make "missing packages" error message more descriptive

Currently there exist confusion due to Pursuit now showing package
sets that include a package. IOW it always looks as if every package
on Pursuit available on every "package set" from the registry. As
result, when a user receives error message from Spago that says that a
package wasn't found, unless a user already knows this situation, they
unlikely would think it's a problem on the remote side rather than
with syntax or something similar.

But on the `spago` side we can reduce confusion by showing a user
a message hinting that it is possible to enlist available packages
with `spago ls packages`.

Fixes: purescript#1287

src/Spago/Command/Fetch.purs

@@ -725,7 +725,9 @@ getTransitiveDepsFromPackageSet packageSet deps = do
   when (not (Set.isEmpty errors.cycle)) do
     die $ "The following packages have circular dependencies:\n" <> foldMap printPackageError (Set.toUnfoldable errors.cycle :: Array PackageName)
   when (not (Set.isEmpty errors.notInPackageSet)) do
-    die $ "The following packages do not exist in your package set:\n" <> foldMap printNotInPackageSetError errors.notInPackageSet
+    die $ "The following packages do not exist in your package set:\n"
+          <> foldMap printNotInPackageSetError errors.notInPackageSet
+          <> "To list available packages use `spago ls packages`\n"
   when (not (Set.isEmpty errors.notInIndex)) do
     die $ "The following packages do not exist in the package index:\n" <> foldMap printPackageError errors.notInIndex
   pure packages