Function `bookmark-get-bookmark' ...

1] tests for the existence of a bookmark by checking whether the input
string matches a cons cell. This casts a net much wider than desirable,
and might better be (assoc bookmark-name-or-record bookmark-alist).

2] returns only a string value on error, without passing nil back to the
calling function.

My usage case is that I was scripting a function to find org-mode files
in a directory tree, and add them to a bookmark list if they aren't
already bookmarked according to my naming convention. Originally, I had
tried checking for the existence of a bookmark using a few of the
bookmark.el functions, such as bookmark-get-bookmark, but that looked
inconvenient because they returned string values on failure.

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