By accident I was trying to open a non-existing database file and got
the following message:

	unable to open database file (unable to open database file)

which was not helpful.

By convention the error returned by a function should contain the
context passed to that function - the filename in Open case, the
operation performed and the error description. So

- teach OpenError to print filename of the opened database and the
  operation.
- don't duplicate the code in braces if we have the error message
  with the same text because they are often redundant.
- if we have only error code but not a message, still we have to include
  the filename and operation in the printed context.
- in the Open itself: always return OpenError on handling errors from
  sqlite3_open_v2 even if db == nil - instead of returning only Errno
  without any context.

Now the error in my case is:

	notfound.db: unable to open database file

( @gwenn requested to omit the ": open" (operation) probable because it
  duplicates error detail a bit )

/helped-by @gwenn