Make depot_tools scripts return exit codes properly on all platforms.

Changes:
1. Windows: exit /b %errorlevel% should be used instead of goto :EOF to get valid exit codes during cmd /c <script>.bat invocation.
2. Windows: delayed var expansion is required in update_depot_tools.bat exit code generation.
3. Posix: update_depot_tools returns exit code from update_git_repo function in case of a failure.

A rule of thumb on Windows: goto :EOF should not be used if %errorlevel% must be returned for all possible invocations.

Test case for update_depot_tools changes:
1. Make a change to depot_tools sources that will conflict with next depot_tools update
2. Run update_depot_tools either directly or via gclient
3. Expect a git error is triggered
4. Inspect %errorlevel% or $? depending on platform
5. Expected 1, but the actual result is 0.

Test case for changes in other .bat files:
1. Make a change to depot_tools sources that will conflict with next depot_tools update
2. Run cmd /c gclient
3. Expect a git error is triggered
4. Inspect %errorlevel%
5. Expected 1, but the actual result is 0.

Change-Id: I64459982bcd9cc3db1319a9b39224b7a7af8c5aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3688632
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Aleksey Khoroshilov <akhoroshilov@brave.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

gclient.bat

@@ -10,8 +10,8 @@ IF "%DEPOT_TOOLS_UPDATE%" == "0" GOTO :CALL_GCLIENT
 :: Synchronize the root directory before deferring control back to gclient.py.
 call "%~dp0update_depot_tools.bat" %*
 :: Abort the script if we failed to update depot_tools.
-IF %errorlevel% NEQ 0 (
-  goto :EOF
+IF %ERRORLEVEL% NEQ 0 (
+  exit /b %ERRORLEVEL%
 )
 
 :CALL_GCLIENT