Refine pattern for Depthcharge TFTP error messages

Refine pattern used to match Depthcharge TFTP errors. This prevents
LAVA from wrongly matching similar errors in Coreboot's late phase
output.

Signed-off-by: Laura Nao <laura.nao@collabora.com>
19 jobs for wip/depthcharge-error-detection in 17 minutes (queued for 13 seconds)
latest
Status Name Job ID Coverage
  Test
passed dispatcher-debian-10 #159617
amd64

00:02:31

passed dispatcher-debian-11 #159619
amd64

00:02:31

passed server-debian-10 #159618
amd64

00:02:49

passed server-debian-11 #159620
amd64

00:05:00

 
  Analyze
passed bandit-sast #159621
amd64-dind

00:00:53

passed black #159624
amd64

00:00:45

passed code_quality #159625
amd64

00:00:24

passed codespell #159630
amd64

00:00:21

passed coverage #159626
amd64

00:06:49

60.98%
passed dockerfiles #159627
amd64

00:00:17

passed eslint-sast #159622
amd64-dind

00:00:24

passed pylint #159629
amd64

00:02:38

passed schemas #159628
amd64

00:00:33

passed semgrep-sast #159623
amd64-dind

00:00:50

 
  Build
passed debian/10 #159632
amd64

00:01:47

passed debian/11 #159633
amd64

00:01:48

passed doc #159631
amd64

00:00:38

passed docker-amd64-dispatcher #159634
amd64-dind

00:04:30

passed docker-amd64-server #159635
amd64-dind

00:04:57