fix crash in autotest_quickmerge when downgraded package doesn't exist
fix crash in autotest_quickmerge when downgraded package doesn't exist

This CL catches a ValueError exception that gets thrown when trying to
get a portage API handle on a non-existant package, and returns this as
a failure to downgrade the given package rather than causing
autotest_quickmerge to crash.

BUG=chromium:239649
TEST=Manually deleted autotest-telemetry package from portage database.
Ran autotest_quickmerge, saw a warning about being unable to downgrade
package rather than seeing autotest_quickmerge crash.

Change-Id: I9e93edaf7d4fa17b176d259adf9934ede362dccc
Reviewed-on: https://gerrit.chromium.org/gerrit/57500
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite