Loads and returns the {bad: good} dictionary stored in the common
		errors data file. Note that the dict's keys are single characters while
		the values do not have to be. The method also asserts that all the
		values are valid IPA strings.

def _load_common_err_data(self, common_err_data_path):
		"""
		Loads and returns the {bad: good} dictionary stored in the common
		errors data file. Note that the dict's keys are single characters while
		the values do not have to be. The method also asserts that all the
		values are valid IPA strings.
		"""
		common_err = {}

		try:
			with open(common_err_data_path, newline='') as f:
				reader = csv.reader(f, delimiter='\t')
				for line in reader:
					if len(line) != 2:
						continue

					try:
						assert line[0] not in common_err
						assert all([char in self.ipa for char in line[1]])
					except AssertionError:
						raise IPADataError('Bad common IPA errors file')

					common_err[line[0]] = line[1]

		except (IOError, ValueError) as err:
			self.log.error(str(err))
			raise IPADataError('Could not open the common IPA errors data file')

		return common_err