Improve handling of ti sdk command failures

[ewanharris]

Describe the Bug

The extension now runs ti sdk list --releases to get the SDK releases. This can fail for numerous reasons such as GitHub rate limiting, or a non-valid SDK semver version being set.

The error reporting needs to be improved so that the error gets reported back to the user. This will need changes in titanium-editor-commons and will also need copying across to the Atom plugin.

See this slack thread for details