Return a 404 instead of a 500 when there's a ValidationError

[sentry-io]

This error came from a program letter, but I think I've seen it for course certificates and program certificates as well.

Sentry Issue: MICROMASTERS-4VF

ValueError: badly formed hexadecimal UUID string
  File "django/db/models/fields/__init__.py", line 2343, in to_python
    return uuid.UUID(**{input_form: value})
  File "uuid.py", line 177, in __init__
    raise ValueError('badly formed hexadecimal UUID string')

ValidationError: ["'ZDhkNmFiOD' is not a valid UUID."]
(19 additional frame(s) were not displayed)
...
  File "django/db/models/lookups.py", line 260, in process_rhs
    return super().process_rhs(compiler, connection)
  File "django/db/models/lookups.py", line 95, in process_rhs
    return self.get_db_prep_lookup(value, connection)
  File "django/db/models/lookups.py", line 187, in get_db_prep_lookup
    [get_db_prep_value(value, connection, prepared=True)]
  File "django/db/models/fields/__init__.py", line 2333, in get_db_prep_value
    value = self.to_python(value)
  File "django/db/models/fields/__init__.py", line 2345, in to_python
    raise exceptions.ValidationError(