blob/fileblob: fix nil underlying error in IfNotExist precondition check

[LeSingh1]

Bug

In both writer.Close() and writerWithSidecar.Close(), the IfNotExist precondition check has two bugs:

1. Nil underlying error: When os.Stat succeeds (meaning the file exists), err is nil. That nil is passed directly to gcerr.New() as the underlying error, resulting in an error chain with no meaningful cause. errors.Is / errors.As callers get no wrapped context.

2. Typo: The message "File already exist" is grammatically incorrect and inconsistent with how memblob reports the same condition.

// Before (both writer.Close and writerWithSidecar.Close)
_, err = os.Stat(w.path)
if err == nil {
    return gcerr.New(gcerrors.FailedPrecondition, err, 1, "File already exist")
    //                                            ^^^-- nil!
}

Fix

Replace the nil underlying error with a descriptive fmt.Errorf, and update the message to be consistent with memblob's phrasing:

// After
_, err = os.Stat(w.path)
if err == nil {
    return gcerr.New(gcerrors.FailedPrecondition, fmt.Errorf("blob %q already exists", w.path), 1, "fileblob: IfNotExist precondition failed")
}

This matches the pattern in memblob where IfNotExist returns:

err := fmt.Errorf("a blob already exists for key %q", w.key)
return gcerr.New(gcerrors.FailedPrecondition, err, 1, "IfNotExist precondition failed")

Testing

The existing drivertest conformance suite exercises IfNotExist writes; this fix ensures the error returned has a non-nil cause when the precondition triggers.

[vangent]

This seems unrelated?

[vangent]

Please rebase to HEAD and force-push, I think that's where the unrelated changes are coming from.