Many of these (mostly in top level commands in commands-packages.js) are
not super well thought out: they use a new "doOrDie" helper to run some
function in a capture and exit if there are any messages. We really
need to get a little more thoughtful about the big picture of error
handling (combining "build" errors, network errors, catalog errors,
etc). But this at least allows the addition of more buildmessage
assertions.
At the very least, this ensures that if you edit a package.js in a local
package while "meteor run" is running, that instead of crashing the tool
it properly shows the buildmessage and lets you fix the issue.
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