Matt Renaud authored May 10, 2020 and Oleg Grenrus committed May 19, 2020

Moved "problem rendering" to CmdErrorMessages module

Additions by Oleg Grenrus:

- There were CommonTargetProblem, but now
  TargetProblem has an extension point so we can have just one type.
  A lot of code is simplified as we don't need to pass in injection
  from CommonTargetProblem to the resulting `err` type.