Since commit 6a6f1d1e (CTest: exit nonzero after
message(SEND_ERROR|FATAL_ERROR), 2020-04-03, v3.19.0-rc1~260^2), `ctest`
no longer runs tests if there are errors before the full set of tests is
defined.  Such errors were previously treated more like warnings.

The change exposed some cases where we were issuing an error message but
proceeding to run tests anyway.  The above commit downgraded one such
case (missing `DartConfiguration.tcl`) to a warning explicitly in order
to restore its former warning-like semantics.

Downgrade the Update step's diagnostic about modified or conflicting
files to a warning for the same reason.

Fixes: #21783