title: opam install Curl failed
body: <p>newbie question:</p>
  
  <p>I have opam version 2.0.5 installed on windows 10 (using cygwin).</p>
  
  <p>I tried to opam install utop</p>
  
  <p>all but 3 out of the 22 packages installed fine, so i don't think it is network issues. 
  What can I do about the 3 that don't install, and why is this happening? (I have tried on two seperate days).</p>
  
  <pre><code>
  &lt;&gt;&lt;&gt; Gathering sources &gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;&lt;&gt;
  [camomile.1.0.2] found in cache
  [charInfo_width.1.1.0] found in cache
  [cppo.1.6.6] found in cache
  [dune.2.1.2] found in cache
  [dune-configurator.2.1.2] found in cache
  [dune-private-libs.2.1.2] found in cache
  [lambda-term.2.0.3] found in cache
  [lwt.5.1.1] found in cache
  [lwt_log.1.1.1] found in cache
  [lwt_react.1.1.3] found in cache
  [mmap.1.1.0] found in cache
  [ocamlbuild.0.14.0] found in cache
  [ocplib-endian.1.0] found in cache
  [result.1.4] found in cache
  [WARNING] ocamlfind.1.8.1: download of
            http://download.camlcity.org/download/findlib-1.8.1.tar.gz failed
            (http://download.camlcity.org/download/findlib-1.8.1.tar.gz (Curl
            failed: "D:\\cygwin64\\bin\\curl.exe --write-out %{http_code}\\n
            --retry 3 --retry-delay 2 --user-agent opam/2.0.5 -L -o
            D:/cygwin64/home/ysg4206/.opam/ocaml-variants.4.07.1+mingw64c/.opam-sw
  itch/sources/ocamlfind.1.8.1/findlib-1.8.1.tar.gz.part
            http://download.camlcity.org/download/findlib-1.8.1.tar.gz" exited
            with code 28)), trying mirror
  [utop.2.4.3] found in cachep] [topkg.1.0.1: http]
  Processing 21/22: [ocamlfind.1.8.1: http] [topkg.1.0.1: htt
  [zed.2.0.4] found in cache
  [ERROR] The sources of the following couldn't be obtained, aborting:
            - ocamlfind.1.8.1: Curl failed
            - react.1.2.1: Curl failed
            - topkg.1.0.1: Curl failed
  
  
  ysg4206@YSG4206 ~
  $ opam --version
  2.0.5
  
  </code></pre>