dl: add go1.2.2, go1.3.3 and go1.4.3

[perillo]

These versions don't have the SH256 checksum available, however the SHA1 checkum is listed on https://golang.org/dl, as an example https://golang.org/dl/#go1.4.3.

In order to support them, it is necessary to print a warning in case the SHA256 is not available, instead of terminating the program.
Here is an example of the warning from my CL (not yet submitted):

Warning: SHA256 Checksum not found: https://storage.googleapis.com/golang/go1.4.3.linux-amd64.tar.gz.sha256: 404 Not Found
The SHA1 Checksum can be found on https://golang.org/dl/#go1.4.3

[toothrot]

/cc @golang/release

[FiloSottile]

We can and should just replace SHA1 hashes with SHA256 hashes for older releases, instead.

[perillo]

@FiloSottile, any news about adding SHA256 hashes for older releases?

[dmitshur]

@perillo It's unlikely we'll have time for this while in the Go 1.17 freeze, but perhaps afterwards.

[perillo]

@dmitshur, well, it is not really an important change. Thanks for the reply.