Exit script/build immediately upon error

[razzius]

Without this change, errors in the build process do not cancel the build, leading to additional errors that can obscure the cause of the build failure.

When I attempted to build planck locally lacking the jar command, the following happened:

planck $ ./script/build
Creating an optimized build, using build cache to speed up build.
Specifying --fast skips optimizations and takes less than two minutes.
Fetching Google Closure Compiler...
Fetching Google Closure Library...
script/get-closure-library: line 17: jar: command not found
script/get-closure-library: line 23: jar: command not found
script/get-closure-library: line 37: cd: planck-cljs/lib/closure/goog: No such file or directory
script/get-closure-library: line 64: cd: planck-cljs/lib/third_party/closure/goog: No such file or directory
find: ‘./var/spool/cron/crontabs’: Permission denied
find: ‘./var/spool/cups’: Permission denied
find: ‘./var/spool/rsyslog’: Permission denied
find: ‘./var/spool/postfix/bounce’: Permission denied
...

It appears that after jar was missing, a cd command didn't work and it took me to the root directory, where further commands attempted to read unrelated directories.

With this change, the build errors immediately once jar fails:

planck $ ./script/build
Creating an optimized build, using build cache to speed up build.
Specifying --fast skips optimizations and takes less than two minutes.
Fetching Google Closure Compiler...
Fetching Google Closure Library...
script/get-closure-library: line 19: jar: command not found
planck $

This seems like preferable behavior to me, but it might lead to issues if other commands depended on the build script to continue through some types of errors. I'm open to alternative approaches to set -e.

It's also worth considering whether such a setting should be in every bash script, if it's considered an improvement.

[mfikes]

I think the unit tests are failing simply because I had recently shut down an HTTP echo server that the HTTP tests depended on.

[mfikes]

Re-started the echo HTTP server and re-ran unit tests, which are passing now.