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