Problem: Tabline is not always redrawn when something that is used in
'tabline' changes.
Solution: Add ":redrawtabline" so that a plugin can at least cause the
redraw when needed.
Problem: When using try/catch in 'tabline' it is still considered an
error and the tabline will be disabled.
Solution: Check did_emsg instead of called_emsg. (haya14busa, closes#746)
