[reclient] update error message for missing reclient binaries/configs

`Developer builds with reclient are not yet supported` is misleading.
This CL updates the message to help users to resolve the issues.

Bug: b/294933473
Change-Id: Ibce2854e25b8994af58011f4e7661f467fe910e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4755000
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>

reclient_helper.py

@@ -196,9 +196,9 @@ def build_context(argv, tool):
   reclient_cfg = find_reclient_cfg()
   if reclient_bin_dir is None or reclient_cfg is None:
     print(("Build is configured to use reclient but necessary binaries "
-           "or config files can't be found.  Developer builds with "
-           "reclient are not yet supported.  Try regenerating your "
-           "build with use_goma in place of use_remoteexec for now."),
+           "or config files can't be found.\n"
+           "Please check if `download_remoteexec_cfg: True` custom var is set"
+           " in `.gclient`, and run `gclient sync`."),
           file=sys.stderr)
     yield 1
     return