Fix dbapi error to comply with pep

[Simon-Chenzw]

• fixes Sqlalchemy can't catch asynch's error #319

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

[Simon-Chenzw]

It changes the native and http and asynch.
I am a little confused about the native and http.
The native wraps the error in DatabaseException. Does it ensure that all errors are caught as DatabaseException? Can we delete DatabaseException?
For http, what is the basic error? Exception may be too wide.

[xzkostyan]

Why do we change Exception to DatabaseException? Ordinary exceptions can be thrown as well.

[Simon-Chenzw]

I'm just a little confused about what DatabaseException does.
If the ordinary exceptions is also thrown, then it should be Exception or clickhouse_driver.Error?

[Simon-Chenzw]

Updated.
Will only change asynch dbapi error.
By the way,

class Error(Exception):
    pass

This does not mean "catch all exceptions". This creates a new exception called "Error", but no one will ever try to raise it.

[Simon-Chenzw]

If you agree to this change, I will write a unittest ASAP.

[xzkostyan]

It's better to write

from asynch.errors import ClickHouseException
...
    Error = ClickHouseException

Yes, please write a test for this fix.

[coveralls]

coverage: 86.276% (-0.005%) from 86.281%
when pulling 0a055c7 on Simon-Chenzw:master
into 523559e on xzkostyan:master.

[Simon-Chenzw]

Test added, sorry for the wait.
I think it's a breaking change so we need to update the changelog?