Improve error message when a toolchain can't be found

When non-Googlers try to build Chromium for the first time they get an
error saying:

  Please follow the instructions at https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md

However those instructions are quite long and it's not obvious to some
users which part of the instructions they have forgotten. The most
likely is that they didn't set DEPOT_TOOLS_WIN_TOOLCHAIN=0, so this
changes the error message to suggest that directly.

Auto-detecting the correct state for DEPOT_TOOLS_WIN_TOOLCHAIN was
tried in crrev.com/c/1374833 but deemed impractical.

Bug: 907300, angleproject:2712
Change-Id: I45a9f59babe90bbe2137d49c555884d7bbbcf170
Reviewed-on: https://chromium-review.googlesource.com/c/1382942Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

win_toolchain/get_toolchain_if_necessary.py

@@ -509,10 +509,15 @@ def main():
       if sys.platform not in ('win32', 'cygwin'):
         doc = 'https://chromium.googlesource.com/chromium/src/+/master/docs/' \
               'win_cross.md'
+        print('\n\n\nPlease follow the instructions at %s\n\n' % doc)
       else:
         doc = 'https://chromium.googlesource.com/chromium/src/+/master/docs/' \
               'windows_build_instructions.md'
-      print('\n\n\nPlease follow the instructions at %s\n\n' % doc)
+        print('\n\n\nNo downloadable toolchain found. In order to use your '
+              'locally installed version of Visual Studio to build Chrome '
+              'please set DEPOT_TOOLS_WIN_TOOLCHAIN=0.\n'
+              'For details search for DEPOT_TOOLS_WIN_TOOLCHAIN in the '
+              'instructions at %s\n\n' % doc)
       return 1
     print('Windows toolchain out of date or doesn\'t exist, updating (Pro)...')
     print('  current_hashes: %s' % ', '.join(current_hashes))