Fix error handling in scap scripts

(bug 50347) set -e has various pitfalls, most relevantly:

(set -e; c1 ; c2) || c3

will execute both c1 and c2 even if c1 fails.

Avoid the use of set -e, instead use perl-style explicit error handling
via shortcut evaluation, as suggested by Ori. Add an explicit "exit 0"
in cases where set -e is not used, and the standard shell behaviour of
passing through the exit status of the last command is undesirable.

Most importantly, scap will abort if mergeMessageFileList.php fails.

Change-Id: I554b79db3338145e6055c5b732da92929a621d13