Better error message on missing language in autogenerated config

Also comment in template

serena_config.template.yml

@@ -24,14 +24,12 @@ gui_log_level: 20
 # minimum log level for the GUI log window (10 = debug, 20 = info, 30 = warning, 40 = error)
 
 
-# Add your list of projects here (which you can switch between using "activate_project").
-# Every list item must be a path to the project's root directory (e.g. "/path/to/myproject"), with the
-# configuration file being located in "/path/to/myproject/.serena/project.yml", or a dict mapping
-# project_name to project_path (to avoid name collisions).
-# You can activate a project by its name using the "activate_project" command.
-# If the name is not explicitly set, the name of the project will be the name of the directory 
-# ("myproject" in the example above).
-# Note: comments in the block below are not preserved when saving the config file,
-# so we recommend to not have any comments directly below the "projects" key.
+# The list of registered projects (which you can switch between using "activate_project").
+# MANAGED BY SERENA, KEEP AT THE BOTTOM OF THE YAML AND DON'T ADJUST WITHOUT NEED (you may add paths though)
+# Whenever a project is activated (either by using the tool or by starting the server with --project),
+# the project's path will be added here and you will be able to activate it by its name in the future.
+# By default, the project's name will be the name of the directory containing the project, but you may change it
+# by changing the entry in the (autogenerated) `.serena/project.yml` file.
+# Note: make sure there are no name collisions in the names of registered projects
 projects:
   - /path/to/myproject

src/serena/agent.py

@@ -109,6 +109,12 @@ class ProjectConfig(ToStringMixin):
             raise FileNotFoundError(f"Project root not found: {project_root}")
         project_name = project_name or project_root.name
         language_composition = determine_programming_language_composition(str(project_root))
+        if len(language_composition) == 0:
+            raise ValueError(
+                f"Failed to autogenerate project.yaml: no programming language detected in project {project_root}. "
+                f"You can either add some files that correspond to one of the supported programming languages, "
+                f"or create the file {os.path.join(project_root, cls.rel_path_to_project_yml())} manually and specify the language there."
+            )
         # find the language with the highest percentage
         dominant_language = max(language_composition.keys(), key=lambda lang: language_composition[lang])
         result = cls(