fail github actions if notebook execution fails (xarray-contrib#80)

* use --warning-is-error when building book

Author: scottyhq

.github/workflows/main.yaml

@@ -13,9 +13,6 @@ permissions:
 jobs:
   build-and-deploy:
     runs-on: ubuntu-20.04
-    defaults:
-      run:
-        shell: bash -l {0}
 
     steps:
       - name: Checkout repository
@@ -31,8 +28,10 @@ jobs:
       - uses: ./.github/actions/setupconda
 
       - name: Build JupyterBook
+        # NOTE: login shell activates conda environment
+        shell: bash -l {0}
         run: |
-          jupyter-book build .
+          jupyter-book build ./ --warningiserror --keep-going
 
       - name: Dump Build Logs
         if: always()