fix: check the json switcher file

[12rambau]

When adding a json switcher using an url I added a few test on the file itself to make sure it's not malformed in order to help developer track down bugs.

Fix #901

[12rambau]

sorry for the noise, some of the errors where only occuring in the github build.

Some explanations:

• checking if there is a json-url key
• check that there is a version key
• find the file using request if the file is distant and read it from the source folder if it's a local one
• check the content of the switcher file and make sure that all elements have a "url" and "version" key
• if anything goes wrong raise an error

[choldgraf]

Looks good to me - one quick question in there