[BUGFIX] Message about missing db credentials is misleading

[BUGFIX] Message about missing db credentials is misleading

If persistent connections are not allowed by MySQL the error message "Can only
select database if username/password/host is correctly set first." is shown.
Add an explanation that $TYPO3_CONF_VARS['SYS']['no_pconnect'] must be set 0
if persistent connections are not allowed.

Change-Id: I28f373c1e771f00d1584eff5080bfa51cf48ed96
Fixes: #29245
Releases: 4.6
Reviewed-on: http://review.typo3.org/4633
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert