Clarify log message

[aeisenberg]

This should cause less confusion and annoyance from users.

[jcreedcmu]

Improving this message makes sense, but isn't this dialog still presenting a
Yes/No question as to whether the extension should open the file
externally? I would expect maybe we want something like:

VS Code does not allow extensions to open files >50MB. This file
exceeds that limit. Do you want to open it outside of VS Code?

You can also try manually opening it inside VS Code by selecting
the file in the file explorer and dragging it into the workspace.

[aeisenberg]

That sounds better.

[MathiasVP]

I definitely like this message better! Thanks for clarifying it.