Catch exceptions in exec callback to ensure Promise is rejected

[chenhuang444]

What

Adds try-catch block to exec callback for machineId()

Why

A Promise has an implicit try-catch reject logic. However, the callback from exec is not handled by this implicit try-catch. Thus any exceptions thrown in expose() will not be caught and the promise will not be rejected.

I noticed this when some of my users had a TypeError: Cannot read property 'replace' of undefined error on sentry

Risk

Low: simple try catch block, does not change original resolve, reject behavior

[chenhuang444]

Resolves this issue #50