isolate ErrorDuringExecution rescue

it only should be rescued for the `curl` invocation
Improve error checking and messages on Cask load

Other Cask sources ultimately invoke the `load` method in the
abstract class Cask::Source::PathBase, so these changes apply
to all other sources.
abstract none_string in doctor
abstract error_string in doctor
render empty doctor values with <NONE>
add locale variables to doctor

references: #4677
move notfound_string to method
move privledged_uid to method in doctor
move fq_default_tap to method in doctor
move alt_taps to method in doctor
move default_cask_count to method in doctor
move homebrew_origin to method in doctor
warning on old-style tap paths in doctor
add render_env_var in doctor

add quoting
add PATH to doctor
whitespace
add $CHRUBY_VERSION to doctor
add $SHELL to doctor
remove issues_url and nullcommand from help output

necessitated by Homebrew's switch to Ruby 2.x
Fixes #4728
Fixes #4726
Update bit-slicer.rb