Update generic "not detectable" error to include support email

[emmambd]

What is the issue and why is it an issue?

As referenced in #140, the "undetectable" error is very generic and makes it difficult for users to know how to solve the problem: "Sorry, this version is not yet implemented or not detectable !"

While providing more specific error messages is a larger issue, a quick minor improvement would be to:

• Specify that a new version cannot be found, potentially with some quick suggestions for what the producer can check (404 URL for example?)
• Providing a support email to follow up with questions

Replacement text:

Sorry, the validator failed to parse this file. Please check that the URL is available and points to a gbfs.json file. If you have any questions, please write to sharedmobility@mobilitydata.org. Thank you

Please describe some potential solutions you have considered (even if they aren’t related to GBFS).

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