Improve error checking messages: add position and use pprint
Improve error checking messages: add position and use pprint

pfctdayelise/pytest,nicoddemus/pytest,txomon/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,alfredodeza/pytest,markshao/pytest,ddboline/pytest,The-Compiler/pytest,The-Compiler/pytest,davidszotten/pytest,hackebrot/pytest,pytest-dev/pytest,Akasurde/pytest,nicoddemus/pytest,tomviner/pytest