Invalid cron schedule does not include positional information

[alecthomas]

I typed in an erroneous cron schedule 0 0 0 0 0 and got:

error: build and deploy failed for module "invoicing": failed to build module "invoicing": failed to extract module schema: value 0 out of allowed day of month range of 1-31

The LSP wasn't able to do anything with this, so as a user I didn't know there was an error until I looked at the FTL output, largely defeating the purpose of the VSCode extension.

Two tasks:

1. The schema for the cron task has positional information, so it should be surfaced.
2. In the more general case of errors with no positional information, the VSCode extension should pop up a notification.