Should not show error on invalid buffers

[MattesGroeger]

Right now this error is shown when trying to remove bookmarks in invalid (non-existing) buffers:

Error detected while processing function ClearAllBookmarks..<SNR>55_remove_all_bookmarks..<SNR>55_bookmark_remove..bm_sign#del:
line    2:
E158: Invalid buffer name: Path/to/not/existing/file