Exit MarkBind with nonzero exit code on fatal error

[Xenonym]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Bug fix

Resolves #651.

What is the rationale for this request?
MarkBind should exit with a nonzero return code (eg. 1) to indicate an error condition. This is especially important for running MarkBind in an automated context where other programs or scripts are relying on the exit code of MarkBind to detect errors (eg. as part of a custom build script, or Travis CI automation).

What changes did you make? (Give an overview)
I created a common error handler handleError to both log an error and set process.exitCode = 1, ensuring that MarkBind exits with a nonzero exit code on error.

Testing instructions:

1. Run a MarkBind command that will exit on an error (eg. markbind build on a directory without a site.json). MarkBind should exit with a nonzero exit code.
   • In Bash: echo $?
   • In PowerShell: $LASTEXITCODE

[yamgent]

LGTM

[yamgent]

@Xenonym need you to resolve conflicts.

[Xenonym]

@yamgent merge conflicts resolved!