Provide much better error messages

- When processes fail, they actually print a failure message
  on the user's terminal
- Regardless of success or failure, we print all output to
  /opt/tljh/installer.log

This should make debugging people's issues *much* easier, since
we can actually see the output of failing commands rather than
having to guess.

tljh/installer.py

@@ -20,6 +20,7 @@ from tljh import (
     systemd,
     traefik,
     user,
+    utils
 )
 from .config import (
     CONFIG_DIR,
@@ -33,7 +34,6 @@ from .yaml import yaml
 
 HERE = os.path.abspath(os.path.dirname(__file__))
 
-
 logger = logging.getLogger("tljh")
 
 def ensure_node():
@@ -170,7 +170,7 @@ def ensure_jupyterlab_extensions():
         '@jupyterlab/hub-extension',
         '@jupyter-widgets/jupyterlab-manager'
     ]
-    subprocess.check_output([
+    utils.run_subprocess([
         os.path.join(USER_ENV_PREFIX, 'bin/jupyter'),
         'labextension',
         'install'