Update install.php5

Defaulting to MySQL here is really dumb. There is an explicit type as an argument so falling back to mysql could result in "I couldn't write to path ....db" despite the real error that the include of the sqlite3/code/SQLiteDatabaseConfigurationHelper.php failed for some reason.

Other uses of getDatabaseConfigurationHelper also need a similar error handler.
Merge pull request #2778 from grooverdan/patch-1

Update install.php5 - error handling on getDatabaseConfigurationHelper