Print error when downloading file error inside install script

[caroline-suse-rancher]

This issue is to track the work done in #6874 by user @if-nil

When an error occurs in the download function of the installation script, the program will simply exit due to the effect of 'set -e':

ubuntu@test:~$ ./install.sh
[INFO]  Finding release for channel stable
[INFO]  Using stable as release
[INFO]  Downloading hash https://github.com/k3s-io/k3s/releases/download/stable/sha256sum-amd64.txt
ubuntu@test:~$

This change can indicate a download error:

ubuntu@test:~$ ./install.sh
[INFO]  Finding release for channel stable
[INFO]  Using stable as release
[INFO]  Downloading hash https://github.com/k3s-io/k3s/releases/download/stable/sha256sum-amd64.txt
[ERROR]  Download failed
ubuntu@test:~$

[VestigeJ]

Confirmed previous behavior
//previous install.sh missing set values in download() function
$ sudo INSTALL_K3S_VERSION=BAD_VERSION_INTENTIONALLY INSTALL_K3S_EXEC=TRAITOR ./install-k3s.sh

[INFO]  Using BAD_VERSION_INTENTIONALLY as release
[INFO]  Downloading hash https://github.com/k3s-io/k3s/releases/download/BAD_VERSION_INTENTIONALLY/sha256sum-amd64.txt

novel behavior when passing bogus information to the script

$ sudo INSTALL_K3S_VERSION=BAD_VERSION_INTENTIONALLY INSTALL_K3S_EXEC=TRAITOR ./install-k3s.sh

[INFO]  Using BAD_VERSION_INTENTIONALLY as release
[INFO]  Downloading hash https://github.com/k3s-io/k3s/releases/download/BAD_VERSION_INTENTIONALLY/sha256sum-amd64.txt
[ERROR]  Download failed