[Windows] Improve et's error if gclient has never been run (#167956)

Mirrors et's shell error here:

https://github.com/flutter/flutter/blob/69da944f9ec3fa20c3026e9a7d3ccb7c1458851c/engine/src/flutter/bin/et#L55-L58
https://dart.googlesource.com/external/github.com/flutter/flutter/+/3cc51e90ca54c4c41064956cae25fa9aef3ca023

DEPS

diff --git a/DEPS b/DEPS
index 52d49ce..4b4c21c 100644
--- a/DEPS
+++ b/DEPS
@@ -236,7 +236,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '9831fc4e9842c377f6f7747663da3bde3e76db5d',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '83a94d9055757366ec15639edcefe4bab7360f32',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '3cc51e90ca54c4c41064956cae25fa9aef3ca023',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index 66e4e08..60b54bd 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"83a94d9055757366ec15639edcefe4bab7360f32",
+  "flutter":"3cc51e90ca54c4c41064956cae25fa9aef3ca023",
   "flutter/engine/src/flutter/third_party/dart":"9831fc4e9842c377f6f7747663da3bde3e76db5d"
 }
\ No newline at end of file