Make it optional to install the extension for lab

Some versions of lab used by repo2docker are not compatible with the
offlinenotebook extension. We try to install the extension and if it
fails we move on without raising an error.

repo2docker/buildpacks/conda/install-miniconda.bash

@@ -54,8 +54,9 @@ conda env create -p ${NB_PYTHON_PREFIX} -f /tmp/environment.yml
 # after the server connection has been lost
 # This will install and enable the extension for jupyter notebook
 ${NB_PYTHON_PREFIX}/bin/python -m pip install https://github.com/manics/jupyter-offlinenotebook/archive/7ba3520.zip
-# and this installs it for lab
-${NB_PYTHON_PREFIX}/bin/jupyter labextension install jupyter-offlinenotebook
+# and this installs it for lab. Keep going if the lab version is incompatible
+# with the extension
+${NB_PYTHON_PREFIX}/bin/jupyter labextension install jupyter-offlinenotebook || true
 
 # empty conda history file,
 # which seems to result in some effective pinning of packages in the initial env,