signing-preflight.sh: fix DRY_RUN mode

This script didn't work right with DRY_RUN=true. Fixes:

* Don't "set -e" otherwise arithmetic evaluations cause the script to
  exit
* Exit 0 on error when DRY_RUN is enabled

Change-Id: I7d58ea0a2de1ed3e645f66b12c87798e07bfc00a
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>

scripts/signing-preflight.sh

@@ -6,8 +6,7 @@
 # SPDX-License-Identifier: Apache-2.0
 #
 
-set -e
-source $(dirname "$0")/lib/job_utils.sh
+source $(dirname "$0")/lib/job_utils.sh || exit 1
 
 require_job_env BUILD_HOME
 
@@ -152,6 +151,6 @@ if [[ ${validation_errors} -eq 0 ]]; then
     exit 0
 else
     error "Found ${validation_errors} pre-flight check error(s)"
-    exit 1
+    $DRY_RUN && exit 0 || exit 1
 fi