fix: better error message when calling push/replaceState before router is initialized

[PatrickG]

"fixes" #11466 and #11492

Not sure if this needs a test.
IMO, this error should be thrown in production as well. Any opinions?

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[kevinflint]

IMO, this error should be thrown in production as well. Any opinions?

It may not be possible to know whether the router is initialized when calling pushState.
It would be nice for there to be a way to tell if it was safe to call, e.g. when writing utility code.

Write now I'm doing something like this in an app codebase:

// Bug: Handle 'undefined' root if pushState is called during on mount.
// See https://github.com/sveltejs/kit/issues/11466 for details.
try {
    pushState(...);
} catch (error) {
    // Log the error as a warning, but don't re-throw it.
    log.warn('Ignoring failed call to `pushState`', error);
}