Guess project root and warn user

Partially closes #230

Author: stefanv

spin/__main__.py

@@ -2,6 +2,7 @@
 import importlib
 import importlib.util
 import os
+import pathlib
 import sys
 import textwrap
 import traceback
@@ -19,8 +20,26 @@
 else:
     import tomli as tomllib
 
+
 click.Context.formatter_class = ColorHelpFormatter
 
+config_filenames = (
+    ".spin.toml",
+    "spin.toml",
+    "pyproject.toml",
+)
+
+
+def _detect_config_dir(path: pathlib.Path) -> pathlib.Path | None:
+    path = path.resolve()
+    files = os.listdir(path)
+    if any(f in files for f in config_filenames):
+        return path
+    elif path.parent != path:
+        return _detect_config_dir(path.parent)
+    else:
+        return None
+
 
 def main():
     # Alias `spin help` to `spin --help`
@@ -38,15 +57,25 @@ def load_toml(filename):
 
     toml_config = collections.ChainMap()
     toml_config.maps.extend(
-        DotDict(cfg)
-        for filename in (
-            ".spin.toml",
-            "spin.toml",
-            "pyproject.toml",
-        )
-        if (cfg := load_toml(filename))
+        DotDict(cfg) for filename in config_filenames if (cfg := load_toml(filename))
     )
 
+    if not toml_config:
+        click.secho(
+            f"Could not load configuration from one of: {", ".join(config_filenames)}",
+            file=sys.stderr,
+            fg="red",
+        )
+        config_dir = _detect_config_dir(pathlib.Path("."))
+        if config_dir:
+            print()
+            print(
+                "Are you running `spin` from the correct directory? Perhaps you'd like to\n"
+            )
+            click.secho(f" $ cd {os.path.relpath(config_dir, '.')}\n")
+            print("and try again.")
+        sys.exit(1)
+
     # Basic configuration validation
     version_query = len(sys.argv) == 2 and (sys.argv[1] == "--version")
 
@@ -55,15 +84,18 @@ def load_toml(filename):
         if "tool.spin" in toml_config:
             spin_config = toml_config["tool.spin"]
             if "tool.spin.commands" not in toml_config:
-                print(
-                    "Error: configuration is missing section [tool.spin.commands]\n",
+                click.secho(
+                    "Error: configuration is missing section [tool.spin.commands]\n"
+                    "See https://github.com/scientific-python/spin/blob/main/README.md\n",
                     file=sys.stderr,
+                    fg="red",
                 )
         else:
-            print(
+            click.secho(
                 "Error: need valid configuration in [.spin.toml], [spin.toml], or [pyproject.toml]\n"
                 "See https://github.com/scientific-python/spin/blob/main/README.md\n",
                 file=sys.stderr,
+                fg="red",
             )
 
     proj_name = (