`yaml.load()` exceptions are not necessarily syntax errors. For
instance, the following YAML source cannot be `load()`ed into a Python
object, but is valid nonetheless:
? - Detroit Tigers
- Chicago cubs
:
- 2001-07-23
? [ New York Yankees,
Atlanta Braves ]
: [ 2001-07-02, 2001-08-12,
2001-08-14 ]
This commit detects syntax errors from `yaml.parse()` exceptions rather
than `yaml.load_all()`.
