Error message handling

[psykokwak4]

Currently we have two separate exception classes in errors module,

class YPYErrorCode(Enum):
    INVALID_UNION_VALUE = 'Cannot translate union value'
    INVALID_ENCODE_VALUE = 'Cannot encode value'
    INVALID_HIERARCHY_PARENT = 'Parent is not set. \
                    Parent Hierarchy cannot be determined'
    INVALID_HIERARCHY_KEY = 'Key value is not set. \
                    Parent hierarchy cannot be constructed'
    INVALID_RPC = 'Object is not an RPC, cannot execute non-RPC object.'
    INVALID_MODIFY = 'Entity is read-only, cannot modify a read-only entity.'
    SERVER_REJ = 'Server rejected request.'
    SERVER_COMMIT_ERR = 'Server reported an error while committing change.'


class YPYError(Exception):
    ''' Base Exception for YDK Errors '''
    def __init__(self, error_code=None, error_msg=None):
        self.error_code = error_code
        self.error_msg = error_msg

    def __str__(self):
        ret = []
        if self.error_code is not None:
            ret.append(self.error_code.value)
        if self.error_msg is not None:
            parser = etree.XMLParser(remove_blank_text=True)
            root = etree.XML(self.error_msg.xml, parser)
            for r in root.iter():
                tag = r.tag[r.tag.rfind('}') + 1 :]
                if r.text is not None:
                    ret.append('\t{}: {}'.format(tag, r.text.strip()))
        return '\n'.join(ret)


class YPYDataValidationError(YPYError):
    '''
    Exception for Client Side Data Validation

    Type Validation\n
    --------
    Any data validation error encountered that is related to type \
    validation encountered does not raise an Exception right away. 

    To uncover as many client side issues as possible, \
    an i_errors list is injected in the parent entity of any entity \
    with issues. The items added to this i_errors list captures the \
    object type that caused the error as well as an error message.

    '''
    pass

The YPYError class deals with server side error, and will populate error messages sent back by the device. The second one is for validation error, it use i_errors to collect as many client side issues as possible. Would it be a good practice to use a single error class, and populate error messages collected by i_errors to the enum YPYErrorCode, to reduce redundancy?

[ghost]

Fixed by #133