Display a user-friendly warning if curl or zstd are missing

PiperOrigin-RevId: 696440060

fetch_databases.py

@@ -19,6 +19,7 @@ import concurrent.futures
 import functools
 import os
 import pathlib
+import shutil
 import subprocess
 import sys
 
@@ -105,6 +106,11 @@ def main(argv=('',)) -> None:
         f' {destination}.'
     ) from e
 
+  if shutil.which('curl') is None:
+    raise ValueError('curl is not installed. Please install it and try again.')
+  if shutil.which('zstd') is None:
+    raise ValueError('zstd is not installed. Please install it and try again.')
+
   # Download each of the files and decompress them in parallel.
   with concurrent.futures.ThreadPoolExecutor(
       max_workers=len(DATABASE_FILES)