Use ExceptT

Move download and downloadDirectory into ExceptT DownloadError IO for more succinct composition.

gbs-downloader.cabal

@@ -118,7 +118,7 @@ library
     , http-client-tls          >=0.3.5.3  && <0.4
     , http-types               >=0.12.3   && <0.13
     , lens                     >=4.0      && <5.3
-    , mtl
+    , mtl                      >=2.2.2    && <2.4
     , network-uri              >=2.6.3    && <2.7
     , servant-client           >=0.16.0.1 && <0.21
     , servant-client-core      >=0.16     && <0.21
@@ -250,7 +250,7 @@ test-suite gbs-downloader-test
     , http-types           >=0.12.3   && <0.13
     , lens                 >=4.0      && <5.3
     , memory               >=0.15     && <0.17
-    , mtl
+    , mtl                  >=2.2.2    && <2.4
     , servant-client       >=0.16.0.1 && <0.21
     , servant-client-core  >=0.16     && <0.21
     , tahoe-chk            >=0.2      && <0.3