Added additional error handling on being unable to access github

[0xicingdeath]

Scenarios:

1. If GitHub is unreachable, the earliest_release should be 0.4.10 instead of 0.4.0.
2. If soliditylang is unreachable, an error should be raised
3. If a user installs all, then all binaries from soliditylang will be installed, with a warning message that 0.4.0-0.4.9 are unavailable
4. If a user tries to fetch available versions and GitHub is unreachable, solc-select will continue execution, with a warning message that 0.4.0-0.4.9 are unavailable

Setting this to a draft PR, so we have time to think about how to test.

[antonper]

In current version also valid_version always tries to query latestRelease. It would be nice if in case github is not accessible the latest version check will not fail.

[CLAassistant]

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