mediawiki.loader: Fix logError() not to print exceptions twice

mediawiki.loader: Fix logError() not to print exceptions twice

Current behaviour results in:

Chrome 65 (initial)
> ▶ [WARN] Error: foo Error: foo
>  at bar
>  at main

Chrome 65 (expanded)
> ▼ [WARN] Error: foo Error: foo
>            at bar
>            at main
>  at logError
>  at main

Safari 11 (initial)
> ▶ [WARN] Error: foo, Error: foo

Safari 11 (expanded)
> ▼ [WARN] Error: foo is undefined
>     * Error: foo is undefined
>         at bar
>         at main

Firefox 57
> [WARN] Error: foo | Error: foo
>                   | Stacktrace:
>                   |   bar
>                   |   main

I don't recall why I did it this way, but it seems redundant.
This commit effectively removes the first "Error: foo"

Change-Id: Idc5dee34ce2b5068e3a2d8800e2b5f6b879525d1