rows_where() and delete_where() fail to throw errors against non-existent tables (fix in v5)

Pointed out in:

• #840

Currently this runs without throwing errors:

python -c 'import sqlite_utils
db = sqlite_utils.Database(memory=True)
print(list(db.table("does_not_exist").rows_where()))                                          
db.table("does_not_exist").delete_where()' 

But this does raise an error:

python -c 'import sqlite_utils
db = sqlite_utils.Database(memory=True)
print(db.table("does_not_exist").count_where())'

Raises:

sqlite3.OperationalError: no such table: does_not_exist

#840 proposed fixing count_where() to return 0. I think the fix is that those other two methods should raise an error, but since this would break compatibility I'm going to hold it for a future v5 version.

Locate the Python implementations of rows_where(), delete_where(), and count_where(), then reproduce the missing-table behavior shown in the issue. Compare how these methods handle nonexistent tables; done means rows_where() and delete_where() raise the appropriate SQLite error, with coverage for both cases, while accounting for the stated v5 compatibility target.