Fixed doc strings and renamed classmethod vars

Fixed doc strings for SgEntity.fromSg(), was old documentation that didn't match
current state.  SgEntity classmethods renamed arg self to cls.
SgEntity.__getitem__ modified error msg when key does not exist

__getitem__ error message better states that the key is not a valid field name.
SgField._valueSg now throws when the Shotgun search doesnt return a value

_valueSg use to return None but now an Exception is raised when the Shotgun
query doesn't return the fields value.  This is most likely because the schema
has changed and the ORM doesn't reflect the new schema.