ensure_gn_version.py: reinstall if tool is missing

It's pretty easy to wedge things currently if you do:
  rm buildtools/linux64/gn

ensure_gn_version.py then gets stuck in a crashing loop where it
tries to run the tool with --version but it always fails w/ENOENT.
So improve the logic to redownload things when they're missing.

Change-Id: Id19c1fdbf9b1ed69930f5a6e3f192e1879ce8d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697947
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676659}

buildtools/ensure_gn_version.py

@@ -18,6 +18,7 @@
 from __future__ import print_function
 
 import argparse
+import errno
 import io
 import os
 import re
@@ -67,14 +68,22 @@ def main():
   except subprocess.CalledProcessError as e:
     print('`%s` returned %d:\n%s' % (cmd_str, e.returncode, e.output))
     return 1
+  except OSError as e:
+    if e.errno != errno.ENOENT:
+      print('`%s` failed:\n%s' % (cmd_str, e))
+      return 1
+
+    # The tool doesn't exist, so redownload it.
+    out = ''
   except Exception as e:
     print('`%s` failed:\n%s' % (cmd_str, e))
     return 1
 
-  current_revision = re.findall(r'\((.*)\)', out)[0]
-  if desired_revision.startswith(current_revision):
-    # We're on the right version, so we're done.
-    return 0
+  if out:
+    current_revision = re.findall(r'\((.*)\)', out)[0]
+    if desired_revision.startswith(current_revision):
+      # We're on the right version, so we're done.
+      return 0
 
   print("`%s` returned '%s', which wasn't what we were expecting."
           % (cmd_str, out.strip()))