Fix bug where setting non-installed GHC unsets current one

lib/GHCup.hs

@@ -346,6 +346,8 @@ setGHC ver sghc = do
   let verBS = verToBS (_tvVersion ver)
   ghcdir                        <- lift $ ghcupGHCDir ver
 
+  whenM (lift $ fmap not $ ghcInstalled ver) (throwE (NotInstalled GHC (ver ^. tvVersion % to prettyVer)))
+
   -- symlink destination
   Settings { dirs = Dirs {..} } <- lift ask
   liftIO $ hideError AlreadyExists $ createDirRecursive newDirPerms binDir
@@ -617,19 +619,16 @@ rmGHCVer :: ( MonadReader Settings m
          -> Excepts '[NotInstalled] m ()
 rmGHCVer ver = do
   isSetGHC <- lift $ fmap (maybe False (== ver)) $ ghcSet (_tvTarget ver)
+
+  whenM (lift $ fmap not $ ghcInstalled ver) (throwE (NotInstalled GHC (ver ^. tvVersion % to prettyVer)))
   dir <- lift $ ghcupGHCDir ver
-  let d' = toFilePath dir
-  exists <- liftIO $ doesDirectoryExist dir
 
-
-  if exists
-    then do
   -- this isn't atomic, order matters
   when isSetGHC $ do
     lift $ $(logInfo) [i|Removing ghc symlinks|]
     liftE $ rmPlain (_tvTarget ver)
 
-      lift $ $(logInfo) [i|Removing directory recursively: #{d'}|]
+  lift $ $(logInfo) [i|Removing directory recursively: #{toFilePath dir}|]
   liftIO $ deleteDirRecursive dir
 
   lift $ $(logInfo) [i|Removing ghc-x.y.z symlinks|]
@@ -653,7 +652,6 @@ rmGHCVer ver = do
     $ hideError doesNotExistErrorType
     $ deleteFile
     $ (baseDir </> [rel|share|])
-    else throwE (NotInstalled GHC (ver ^. tvVersion % to prettyVer))
 
 
 -- | Delete a cabal version. Will try to fix the @cabal@ symlink