Rowan tree with cracked bark and dead branches

[pellgarlic]

Hi,

 

I hope someone will be able to offer some advice on the seriousness of the state of my Rowan tree.

 

About three years ago, I noticed a dead branch on the North side (it had no leaves).
I noticed that a branch had been removed already (I could see the "stump"), did a bit of research into how best to remove it, and did so myself.
(I can sense you all cringing already - I probably should have got a professional to do it, but hindsight is 20-20 :( ).
In subsequent years, I've noticed a couple of other dead branches too, and taken them off also, to the best of my ability.

This year, I started to see that there are cracks in the bark, both on the trunk, and on some higher branches, and that the "stump" from the removal previous to my attempts wasn't looking great.
I can't say for sure when the cracks appeared, they may have even been there before I moved into the house (five years ago).

 

So my question is - is this a serious issue for the tree?
If not, what should I do to try to protect it from further damage?
If it is a serious issue, is the tree saveable? 
If not, does it pose any immediate danger, or might it last for years more?

Thanks in advance for any comments.