version: Fail gracefully if git describe cannot get version.
version: Fail gracefully if git describe cannot get version.

When running the `./tools/cache-zulip-git-version` script on Travis, the script
fails because Travis gets a shallow clone of the repository, and not a full
clone. This commit changes the script to fail gracefully, if we are unable to
get the version information using `git describe`.

When the command fails, it still writes an empty `zulip-git-version` and that
has not been changed to keep creation of the release tarball simple, and
avoiding a check for whether the file has any content. The code that sets
`ZULIP_VERSION` checks whether the contents of the `zulip-git-version` file are
empty, before setting `ZULIP_VERSION`. So, the version should never be set to an
empty string.

kou/zulip,zulip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,shubhamdhama/zulip,punchagan/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,tommyip/zulip,synicalsyntax/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip,punchagan/zulip,tommyip/zulip,shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,tommyip/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,brainwane/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,rishig/zulip,andersk/zulip,rht/zulip,shubhamdhama/zulip,timabbott/zulip,tommyip/zulip,rht/zulip,tommyip/zulip,punchagan/zulip,rht/zulip,showell/zulip,kou/zulip,hackerkid/zulip,kou/zulip,brainwane/zulip,synicalsyntax/zulip,rishig/zulip,kou/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,rishig/zulip,andersk/zulip,synicalsyntax/zulip,showell/zulip,rishig/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,punchagan/zulip,timabbott/zulip,timabbott/zulip,kou/zulip,andersk/zulip,showell/zulip,rishig/zulip,rht/zulip,rht/zulip,rishig/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,tommyip/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip