Raise the sqlite3 import error, not the pysqlite2 one

Raise the sqlite3 import error, not the pysqlite2 one

Fixed the import error raised when a platform
has neither pysqlite2 nor sqlite3 installed, such
that the sqlite3-related import error is raised,
not the pysqlite2 one which is not the actual
failure mode.  Pull request courtesy Robin.

Origin version only print `No module named pysqlite2` even
it's actually the import error of line 337: `from
sqlite3 import dbapi2 as sqlite` which point user
to the wrong debug direction.

It should raise `e.message` as `No module named _sqlite3`.

Change-Id: Idc39cd0d226957fd670859df23a2386dea6eb3cc
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/417