Improve weather/0

[mnishiguchi]

@fhunleth I tentatively prototyped #125 with #60 in mind.

Changes

• extract weather/0 command from Toolshed.HTTP, newly creating Toolshed.Weather
• extract check_app/1 helper from Toolshed.HTTP, newly creating Toolshed.Utils
• catch :exit signal that potentially occurs when :httpc server crashes
• rescue unexpected responses

Notes

• I was able to recreate two types of potential errors on my local machine
  • :https server crashing, which throws :exit
  • :https responds with unexpected response, which is currently anything but {:ok, {_, _, _}}
• I have trouble wording errors so it would be great if you come up with better messages.
• resolves Improve errors from weather #125

[fhunleth]

I think this is good. Thank you!

When we hit errors in our day-to-day use, we can fine tune the error messages if that's needed.