yaml has not attribute `ConstructorError`

[braingram]

Attempts to raise yaml.ConstructorError are included in asdf.yamlutil.

asdf/asdf/yamlutil.py

Line 178 in 45b1661

raise yaml.ConstructorError(

For yaml 6.0 (and perhaps other versions) the ConstructorError attribute does not exist. However, yaml.constructor.ConstructorError does.