Turn overriding command warnings into errors by default

All of the existing devices that have problems with this are setting
BUILD_BROKEN_DUP_RULES to true. Switch the behavior to default this to
an error.

Bug: 77611511
Test: check logs of all downstream build_test targets
Test: attempt to add an overriden command to hikey960 (uses default)
Change-Id: Ia83089f035925cf0c9883a1f593ae2da89fee568