Ignore failure flag when ami datasource can't find ami

#### Description
I would like to add an option for ami datasource that will "ignore failure" when a ami is not found between aws ami's.

#### Use Case(s)

I have 2 cases using same packer file:
- the ami exists
- the ami does not exits, and I want to fall back to hardcoded ami id.
The current version of the plugin fails when the datasource doesn't find any ami by the given filters.

#### Potential References
Error example:
`Error: Datasource.Execute failed: No AMI was found matching filters:`

I will apply a PR for this one and mention it here.