Harden upper-constraints.txt operations

Harden upper-constraints.txt operations

It takes into account the wget exit values and avoids manipulating a
falsy empty file.

It won't help fixing the network outages in LF network which have been
highlighted by false failures in api_check (the wrong snaps package
is installed [1])

[1] https://build.opnfv.org/ci/job/functest-opnfv-functest-healthcheck-latest-api_check-run/100/console

Change-Id: Ic9b75965b1ffb1f7bb342ff216bda9933ca75f0b
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>