EVA-4258 - Metadata fixes

[tcezard]

• Modified the schema enforcement for sheet containing two options: Project and Sample: Used if/then/else to provide the two required alternative.
• Prevented xls->json conversion error when value cannot be cast so that the validator can highlight the source of the mistake more precisely
• Add explanations of the invalid regex errors to make it easier to understand

[apriltuesday]

I didn't know about if/then/else in JSON schemas, that's a nice solution!