Make MetadataWriter more robust

[jeremypw]

• Add some null checks
• Remove asserts and replace with critical messages

The codebase contains a lot of places where the app just terminates due to an assertion failure. This does not seem necessary for production code and these have been replaced with critical messages or throwing an error and the subsequent code either omitted or continued as seems appropriate. If reviewers feel an assertion failure would cause user data corruption on continued use then it should be replaced with a call to AppWindow.panic which at least logs a message before terminating.

There were a few places where a null could theoretically be dereferenced.

It is difficult to know whether assertion failure or null dereferencing could occur in practice as the code is extremely convoluted.

[stsdc]

At this point I'm expecting you also to approve your PR 😄

[jeremypw]

On further consideration it is not necessary to explicitly null check in the blacklist_file functions - the Vala compiler automatically inserts a null check. One disadvantage of relying on this is that is not immediately obvious what is returned if the parameter requirements are not met when the function signature returns something. In fact it seems that boolean functions return false (safe enough) and functions returning a pointer return null (might be unsafe).