Fall back relative time to english if cant find loaded locale

[lkostrowski]

Currently dayjs is not actually "safe" in terms of usage.

I know there is a goal of keeping size limit, however maybe there is a reasonable way to improve code quality and move code compiling to some super-minifier like Closure compiler, without omitting code security and overall readability (variable names etc)?

In this PR I do fallback to english locale in relativeTime plugin, which throws without reasonable message if locale isn't loaded properly. I also show error in console. I have expected behavior like this since I failed loading language in my project and it didn't inform me about it.

Don't know if this is something expected in this library or it's very "raw" deliberately