Handle unexpected proxy query results gracefully

[neclepsio]

When the proxy contacted for the released gopls version list does not
respond following the Go module proxy protocol but return unexpected
contents (e.g. login page) semver parsing will fail, trigger an exception,
and cause the language server client not to start.

Fixes #3204

[hyangah]

Thanks for the fix!
Can you provide the PR title & description? Something like

Handle unexpected proxy query results gracefully

When the proxy contacted for the released gopls version list does not
respond following the Go module proxy protocol but return unexpected
contents (e.g. login page) semver parsing will fail, trigger an exception,
and cause the language server client not to start.
 
Fixes #3204

The linked issue in the title has the full description already, but it's always nice to have a descriptive commit message - unless @ramya-rao-a will manually edit the commit message before merge.

[ramya-rao-a]

Looks like this is your first PR contribution to this project @neclepsio
Thanks & Welcome!