Improve error reporting when the database string is unparsable.

[jlfranklin]

Describe your issue or idea

Under certain conditions, the database string is unparsable and the system returns a cryptic error, such as Fatal error: Class 'DatabaseTasks_' not found in.... Instead of a PHP compilation error, it should say something like this:

Database configuration could not be parsed. This is usually because of a typo in the database string or certain special characters in the database's user name or password, such as the at-sign, colon, semicolon, or slash. Please check the database string in settings.php and try again.

Steps to reproduce (if reporting a bug)

Trigger the bug mentioned in #2231.

Actual behavior (if reporting a bug)

Errors that make sense if you're a core developer, and no sense at all if you're a rookie site builder.

Expected behavior (if reporting a bug)

An error message with enough information for the user to take action.

Relevant version/system information (if applicable)

1.x

[quicksketch]

Thanks @jlfranklin! I tested this out and it helps a lot! I found two additional errors occur when the $database string is not valid, so I filed a rerolled PR at backdrop/backdrop#2053 that combines your code with two more checks, preventing unrelated errors from showing when the database string is not valid.

[quicksketch]

Merged backdrop/backdrop#2053 into 1.x and 1.8.x. Thanks @jlfranklin for working to improve our bootstrap systems!