Give useful error if 'python-gitlab' not found

gl-sak

@@ -36,18 +36,28 @@ of your runtime environment, but if you are in any doubt, just use
 '--authn-token-file' instead.
 """
 
-# To get started:
-# 
-#   $ git clone git@gitlab.com:python-gitlab/python-gitlab.git
-#   $ cd python-gitlab
-#   $ python -m venv .venv
-#   $ source .venv/bin/activate
-#   venv$ pip install -e .
-# 
-# See https://python-gitlab.readthedocs.io/en/stable/api-usage.html
-
-import gitlab
 import argparse
+import sys
+try:
+    import gitlab
+except ImportError:
+    print("""ERROR: Python GitLab API bindings not found.
+
+Please install 'python-gitlab' and make it available in your Python
+run-time environment.  This recipe should work for most people:
+
+  $ git clone git@gitlab.com:python-gitlab/python-gitlab.git
+  $ cd python-gitlab
+  $ python -m venv .venv
+  $ source .venv/bin/activate
+  venv$ pip install -e .
+  venv$ cd .../wherever/gl-sak/lives
+  venv$ ./gl-sak --help
+ 
+(See https://python-gitlab.readthedocs.io/en/stable/api-usage.html
+for more about the Python GitLab API package.)""")
+    sys.exit(1)
+
 
 def main():
     # Set up options and arguments