installer should be aware of more errors such as "Table 'drupal_install_test' already exists"

When on installs drupal, if there was en error in the mysql's grants, the installer wrongly complain about being unable to create table, when it should just drop the table it previously created instead of reporting the following:

Failed to create a test table on your MySQL database server with the command 'CREATE TABLE drupal_install_test (id int NULL)'. MySQL reports the following message: Table 'drupal_install_test' already exists.