Zebra: Ignore the failure of startup intf lookup.

Zebra: Ignore the failure of startup intf lookup.

In startup, zebra would dump interface information from Kernel in 3
steps w/o lock: step1, get interface information; step2, get interface
ipv4 address; step3, get interface ipv6 address.
If any interface gets added after step1, but before step2/3, zebra
would get extra interface addresses in step2/3 that has not been added
into zebra in step1. Returning error in the referenced interface lookup
would cause the startup interface retrieval to be incomplete.

Signed-off-by: Yuan Yuan <yyuanam@amazon.com>