add: ignore only ignored files

"git add foo bar" adds neither foo nor bar when bar is ignored, but dies
to let the user recheck their command invocation. This becomes less
helpful when "git add foo.*" is subject to shell expansion and some of
the expanded files are ignored.

"git add --ignore-errors" is supposed to ignore errors when indexing
some files and adds the others. It does ignore errors from actual
indexing attempts, but does not ignore the error "file is ignored" as
outlined above. This is unexpected.

Change "git add foo bar" to add foo when bar is ignored, but issue
a warning and return a failure code as before the change.

That is, in the case of trying to add ignored files we now act the same
way (with or without "--ignore-errors") in which we act for more
severe indexing errors when "--ignore-errors" is specified.