[WIP] Check valid external call exception handling

[ckamueller]

Closes #97, and Closes #96 in combination with #98.

• [x] check that call! always occurs outside a do ... catch block
• [ ] check that there is always a call inside a do ... catch block
• [ ] check that call? is only used in an if let ... construct
• [ ] check that an external call return value is never ignored

[ethanluoyc]

Ping me and remove the WIP label when you are ready, I can do the reviews.

[nvgrw]

What is with the title of this PR? Can you please change it to something meaningful related to the ticket

[ckamueller]

Replaced by #128