Controversies in translation for log and exception.

[Mitya Eremeev]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way:
there are controversies regarding translating a string for logging and exception:
1. we translate a string and put it in log and exception
# Log and Raise
    msg = _('Unexpected error message')
    LOG.exception(msg)
    raise RuntimeError(msg)
https://docs.openstack.org/oslo.i18n/latest/user/usage.html

2. we put one not translated string in log and another translated string in exception
For example, do not do this:

# WRONG
msg = _('There was an error.')
LOG.error(msg)
raise LocalExceptionClass(msg)
or this:

# EVEN MORE WRONG
msg = _LE('There was an error.')
LOG.error(msg)
raise LocalExceptionClass(msg)
Instead, use this style:

# RIGHT
LOG.error('There was an error.')
raise LocalExceptionClass(_('An error occurred.'))
https://docs.openstack.org/oslo.i18n/latest/user/guidelines.html

- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 5.1.0.dev6 on 2018-09-12 22:18:36
SHA: 8609dc2c4ceacd2b5b3acb34430de29838a219fa
Source: https://opendev.org/openstack/oslo.i18n/src/doc/source/user/usage.rst
URL: https://docs.openstack.org/oslo.i18n/latest/user/usage.html