show a more useful error message if python environment is not (completely) set up

main.py

@@ -1,7 +1,29 @@
 #!/usr/bin/env python
 
-from sys import exit
 
-from core.cli.main import run_pythagora
+import os.path
+import sys
 
-exit(run_pythagora())
+try:
+    from core.cli.main import run_pythagora
+except ImportError:
+    pythagora_root = os.path.dirname(os.path.dirname(__file__))
+    venv_path = os.path.join(pythagora_root, "venv")
+    requirements_path = os.path.join(pythagora_root, "requirements.txt")
+    if sys.prefix == sys.base_prefix:
+        venv_python_path = os.path.join(venv_path, "scripts" if sys.platform == "win32" else "bin", "python")
+        print("Python environment for Pythagora is not set up.", file=sys.stderr)
+        print(f"Please create Python virtual environment: {sys.executable} -m venv {venv_path}", file=sys.stderr)
+        print(
+            f"Then install the required dependencies with: {venv_python_path} -m pip install -r {requirements_path}",
+            file=sys.stderr,
+        )
+    else:
+        print("Python environment for Pythagora is not completely set up.", file=sys.stderr)
+        print(
+            f"Please run `{sys.executable} -m pip install -r {requirements_path}` to finish Python setup, and rerun Pythagora.",
+            file=sys.stderr,
+        )
+    sys.exit(255)
+
+sys.exit(run_pythagora())