fix: error parsing polkadot version from github API

[AlexD10S]

Close: #138

In pop up parachain we are getting the last Polkadot release from the GitHub API. If there is something wrong with this call (for example: IP blocked by Github due to rate limit), return at least a default version

[evilrobot-01]

What happens if user already has latest version installed, but is then dropped down to an older default version? It will then attempt to clone and build, potentially failing on the clone for the same reason.

[AlexD10S]

What happens if user already has latest version installed, but is then dropped down to an older default version? It will then attempt to clone and build, potentially failing on the clone for the same reason.

If user has latest version and the default version is an older one it will download and build the older one. Even if not installing the latest one, at least we are preventing to crash like in: #138
And for an advanced user that knows what version is the last one, can still specify the version with the flag --relay_chain
The clone has not the same limit as the API, so should be fine. Asked in the Github issue #138 (comment).