bpo-33106: change dbm key deletion error for readonly file from KeyError to dbm.error

[zhangyangyu]

https://bugs.python.org/issue33106

[zhangyangyu]

Something weird is on Mac the manual tells:

The dbm_delete(db, key) function deletes the entry for key. The dbm_delete() function normally returns zero but returns 1 if there was no entry with key in the database or
returns -1 and sets errno if there were any errors.

But it seems not right:

Python 3.6.4 (default, Mar  9 2018, 23:15:03)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import dbm.ndbm
>>> f = dbm.ndbm.open('abc', 'c')
>>> f[b'key'] = b'value'
>>> f.close()
>>> f = dbm.ndbm.open('abc', 'w')
>>> del f[b'not exist']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: b'not exist'

http://pubs.opengroup.org/onlinepubs/009695399/functions/dbm_delete.html

[bedevere-bot]

@zhangyangyu: Please replace # with GH- in the commit message next time. Thanks!