Replace occurrences of `throw new Error(...)` with `unreachable(...)` where applicable

Especially in abstract methods, we would like to use unreachable(...) because it’s a more explicit way to indicate that a fail state of the application is reached. Let’s ensure that all abstract methods use unreachable(...) instead of throwing an error.

Note: we use unreachable(...) only for code paths that cannot happen by design (such as directly calling an abstract method). Errors in parsing can happen if the PDF is corrupted for example, so since that is dependent on the input, we throw a regular error there.

[Snuffleupagus]

Since a number of people have expressed interest in working on this, but no patch has yet been provided, let’s try the following approach to make some headway here: If someone leaves a comment stating that they intend to work on this issue, but doesn’t provide a patch (nor any kind of update) within one week, then this issue will be considered available for another contributor.

[nveenjain]

Can i work on this if this issue is still available?