Use SaveConditionError instead of OperationError in save_condition

[touilleMan]

Hi there,

Working with save_condition in save, I realised using an OperationError exception is too vague when trying to return fine errors to client, example:

class Doc(Document):
    version = fields.IntField(default=1)
    field = fields.StringField(unique=True)

doc_1 = Doc(field='unique').save()
try:
    doc_2 = Doc(field='unique').save() # raises NotUniqueError, subclass of OperationError
except OperationError:
    # Should handle HTTP 400 error
    pass
doc_1.field = 'two'
http_if_match = 42 # Client provide If-Match header in HTTP request
try:
    doc_1.save(save_condition={'version': http_if_match}) # raises OperationError
except OperationError:
    # Should handle HTTP 412
    pass

I solve this by replacing the OperationError triggered by an invalid save_condition by a new SaveConditionError (subclass of OperationError, to mimic the behaviour of NotUniqueError)