Fix soong crashing when there's no baseline file

Soong was referencing the baseline file regardless of whether it was
valid or not.

Also update the lint message to match the one metalava itself prints,
which only suggests whitelisting the error if it's not possible to fix
it properly.

Bug: 154616909
Test: m (with lint error)
Change-Id: I4c6ec327a59aefe74825b4c4719393dd267ba3aa