Detect when MR has a description that is too long

[mschoettle]

Throw an error when the max length of the body is exceeded.

In order for the execution to stop and not continue with the remaining merge requests it is necessary to break here:

node-gitlab-2-github/src/index.ts

Lines 587 to 592 in 6b1d903

	} catch (err) {
	console.error(
	'Could not create pull request: !' + mr.iid + ' - ' + mr.title
	);
	console.error(err);
	}

If this is desired, I can add it.

Fixes #208

[spruce]

Yeah, I think it would be better to stop the migration in that place.

(Later when it is possible to restart a migration it would be even more useful to do so)

[mschoettle]

@spruce I added the change that stops the execution when migrating a merge request fails.