handle product error exceptions better and check the errors in the tests

[gyorb]

During the running of the tests I got these errors, they should be handled better and check why they happened.

19:13:03   File "libcodechecker/server/server.py", line 355, in do_POST
19:13:03     self.__check_prod_db(product)
19:13:03   File "libcodechecker/server/server.py", line 298, in __check_prod_db
19:13:03     raise ValueError(error_msg)
19:13:03 ValueError: 'producttest' database connection failed!

19:13:00 [ERROR 2018-06-07 19:13] - The product None does not exists
19:13:00 [ERROR 2018-06-07 19:13] - The product None does not exists
19:13:00 Traceback (most recent call last):
19:13:00   File "libcodechecker/server/server.py", line 355, in do_POST
19:13:00     self.__check_prod_db(product)
19:13:00   File "libcodechecker/server/server.py", line 270, in __check_prod_db
19:13:00     raise ValueError(error_msg)
19:13:00 ValueError: The product None does not exists