Check exit codes for commands in the browse_build function

[meganschanz]

We had noticed that it was possibly for the sort command in the browse_build function in the index-alphabetic-browse.sh script to fail, but since it was not checking the exit code of that sort command, it was improperly returning an exit code of zero. Instead of just adding the check for that command I thought it wouldn't hurt to have the exit code check on all the commands in that browse_build function.

This is particularly useful for us because we have a wrapper script around index-alphabetic-browse.sh that will alert us when there are failures (i.e. the script exits with a non-zero exit code).

[demiankatz]

Thanks, @meganschanz, this is a useful improvement. I also appreciate the addition of comments!

I don't have the time or inclination to fully port this to the Windows version of the script, but I did at least align the comments between the two versions -- see 3d47b63.