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

Replaces #113

[nvgrw]

Why replace the old one?

[ckamueller]

This requires a lot changes from basic external call semantic checks merged into master.