title: How to revert conda updates when prompt meets unexpected error
body: <p>I have updated Spyder (<code>conda update spyder</code>) this morning using conda on Windows 8 and later when I checked I was unable to launch Spyder and Anaconda Navigator.</p>
  
  <p>I tried updating the package again, but the prompt gave me the following warning and wasn't able to retrieve the package metadata:</p>
  
  <pre><code>WARNING conda.base.context:use_only_tar_bz2(632): Conda is constrained to only
  using the old .tar.bz2 file format because you have conda-build installed, and
  it is &lt;3.18.3.  Update or remove conda-build to get smaller downloads and faster
  extractions.
  </code></pre>
  
  <p>Given that, I tried reverting the entire operation using <code>conda install --revision 9</code> (the previous revision), upon which I ran into the same warning message. This time, the prompt attempted to rollback, but it ultimately crashed with a lengthy report that finished with the following message: </p>
  
  <pre><code>An unexpected error has occurred. Conda has prepared the above report. If submitted,
  this report will be used by core maintainers to improve future releases of conda.
  Would you like conda to send this report to the core maintainers?
  </code></pre>
  
  <p>Attempting either <code>conda update conda-build</code> and <code>conda remove conda-build</code> as suggested by the prompt incurs in the exact same error.</p>
  
  <p>How can I restore my environment since conda locks me inside an error whenever I perform an operation to revert or remove something?</p>
  
  <p>If it helps, this is all that changed in the environment with the update:</p>
  
  <pre><code>2019-07-03 10:40:00  (rev 10)
   ca-certificates  {2019.3.9 (conda-forge) -&gt; 2019.6.16 (conda-forge)}
   certifi  {2019.3.9 (conda-forge) -&gt; 2019.6.16 (conda-forge)}
   conda  {4.6.14 (conda-forge) -&gt; 4.7.5 (conda-forge)}
   cryptography  {2.5 (conda-forge) -&gt; 2.7 (conda-forge)}
   krb5  {1.16.2 (conda-forge) -&gt; 1.16.1}
   libssh2  {1.8.0 (conda-forge) -&gt; 1.8.2 (conda-forge)}
   openssl  {1.0.2r (conda-forge) -&gt; 1.1.1b (conda-forge)}
   pycurl  {7.43.0.2 -&gt; 7.43.0.2}
   pyqt  {5.6.0 (conda-forge) -&gt; 5.9.2 (conda-forge)}
   qt  {5.6.2 -&gt; 5.9.7}
   sip  {4.18.1 (conda-forge) -&gt; 4.19.8 (conda-forge)}
   spyder  {3.3.4 (conda-forge) -&gt; 3.3.5 (conda-forge)}
   spyder-kernels  {0.4.4 (conda-forge) -&gt; 0.5.0 (conda-forge)}
  +conda-package-handling-1.3.10 (conda-forge)
  +libarchive-3.3.3 (conda-forge)
  +lz4-c-1.8.3 (conda-forge)
  +python-libarchive-c-2.8 (conda-forge)
  +xz-5.2.4 (conda-forge)
  +zstd-1.3.3 (conda-forge)
  </code></pre>