replace use of MissingSourceFile with LoadError

First of all, a top level constant, `MissingSourceFile` is just an alias of `LoadError`: https://github.com/rails/rails/blob/39374aa925a7d670b039c0c0c9aa9f4aef19466b/activesupport/lib/active_support/core_ext/load_error.rb#L25

Rails internally uses `MissingSourceFile` only in this particular case: https://github.com/rails/rails/blob/39374aa925a7d670b039c0c0c9aa9f4aef19466b/actionpack/lib/abstract_controller/helpers.rb#L172
and uses `LoadError` in all other places. I know the Rubyists say "diversity is good", but we don't need diversity like this.

Now the `MissingSourceFile` constant could be removed from ActiveSupport, but I don't think we're able to do it right now, because I found several libraries referencing a constant with this name, which might be expecting ActiveSupport to define it.