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Resolves #4566
The SQL query for PostgreSQL/SQlite built at https://github.com/mybb/mybb/blob/39e16eca5ff97bcb82843b306106df827399e25d/css.php#L31-L46 doesn't separate the initial part from subsequent ones with a whitespace character, resulting in a SQL error.
Resolves #4557
Currently, database errors are triggered by calling the `errorHandler::error()` method in database driver classes (through `DB_Base::error()`), making it difficult to intercept and analyze them. Throwing Exceptions instead would allow `errorHandler`...
Resolves #4564 Depends on: #4559
Resolves #4561 May conflict with #4560
Add a [generic error](https://github.com/mybb/mybb/blob/2b5317ab1dabba0417306a29281d84cdb7975d8d/inc/class_core.php#L534) for missing dependencies and trigger it when the Composer-generated `inc/vendor/autoload.php` file does not exist.
Resolves #4559
Add a global exception handler and process exceptions in similar way to errors. MyBB 1.9, and dependencies, are expected to rely on exceptions more often.