The type: fcv_string is been used in couple of different idls other than the idl that defines feature compatibility version document schema  for admin.system.version collection. For e.g, setFeatureCompatibilityVersion cmd  & tenant migration state document have fields uses this fcv_string type. In case parsing those fcv type fields fails, we through a misleading error message.

Running adminDB.adminCommand({setFeatureCompatibilityVersion: "4.9"} on  5.2 binary will throw an error msg as "Invalid value for featureCompatibilityVersiondocument in admin.system.version, found 4.9, expected '5.0' or '5.1' or '5.2'. 

I think we need to make the error message as generic.