open: Fix error message

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

sqlite.go

@@ -40,10 +40,23 @@ type OpenError struct {
 }
 
 func (e OpenError) Error() string {
+	file := e.Filename
+	if file == "" {
+		file = "(temporary)"
+	}
+	s := fmt.Sprintf("%s: ", file)
+	codeErr := e.Code.Error()
 	if len(e.Msg) > 0 {
-		return fmt.Sprintf("%s (%s)", e.Msg, e.Code.Error())
+		s += e.Msg
+		// error code and msg are often redundant but not always
+		// (see sqlite3ErrorWithMsg usages in SQLite3 sources)
+		if codeErr != e.Msg {
+			s += fmt.Sprintf(" (%s)", codeErr)
+		}
+	} else {
+		s += codeErr
 	}
-	return e.Code.Error()
+	return s
 }
 
 // ConnError is a wrapper for all SQLite connection related error.
@@ -250,16 +263,17 @@ func OpenVfs(filename string, vfsname string, flags ...OpenFlag) (*Conn, error)
 	}
 	rv := C.sqlite3_open_v2(cname, &db, C.int(openFlags), vfs)
 	if rv != C.SQLITE_OK {
-		if db != nil { // try to extract futher details from db...
-			err := OpenError{Code: Errno(rv),
-				ExtendedCode: int(C.sqlite3_extended_errcode(db)),
-				Msg:          C.GoString(C.sqlite3_errmsg(db)),
+		err := OpenError{
+			Code:     Errno(rv),
 			Filename: filename,
 		}
+		if db != nil { // try to extract futher details from db...
+			err.ExtendedCode = int(C.sqlite3_extended_errcode(db))
+			err.Msg = C.GoString(C.sqlite3_errmsg(db))
 			C.sqlite3_close(db)
 			return nil, err
 		}
-		return nil, Errno(rv)
+		return nil, err
 	}
 	if db == nil {
 		return nil, errors.New("sqlite succeeded without returning a database")