EZP-20606: Create a float fieldtype as "isSearchable" succeeds

This PR fixes issue https://jira.ez.no/browse/EZP-20606

Validation for ```isSearchable``` was not performed. This implements validation adding error to the ```ContentTypeFieldDefinitionValidationException``` exception in ContentTypeService (BL layer).

Additionally, code flow is changed so that all possible validation of FieldDefinition is performed before throwing ```ContentTypeFieldDefinitionValidationException```.