get rid of unused --regtest-online option
catch any exceptions occuring per build, improve reporting of failed builds
improve error message on failed HTTP request to GitHub API
catch exception occuring during download
don't exit prematurely from build_and_install_software by raising exception, return error message too
fail on first failed build, except when using --test-easyconfigs-pr, create gist with partial log for failed builds
move regtest function to new module easybuild.tools.testing
move posting of PR test report into testing module, fix various remarks
split composing of test report from posting it
switch to using MarkDown as format for test report body