Adding a check for file in convert cli. Earliar it used to raise long…

… error

marimo/_cli/convert/commands.py

@@ -50,6 +50,9 @@ def convert(
     you may need to fix errors like multiple definition errors or cycle
     errors.
     """
+    if not Path(filename).exists():
+        raise click.FileError(filename, "File does not exist")
+
     ext = Path(filename).suffix
     if ext not in (".ipynb", ".md", ".qmd"):
         raise click.UsageError("File must be an .ipynb or .md file")