Improve handling of HTTP errors, properly log $execOutput

[TysonAndre]

Also, $execOutput is an array of output lines, not a string - https://www.php.net/manual/en/function.exec.php

E.g.

• Add a finite number of retries?
• Add a way to skip failed downloads

--2021-09-17 09:24:24--  https://api.github.com/repos/aws/aws-sdk-php/zipball/3b4787381bd747753059a223b801ea5eb7d11780
Resolving api.github.com (api.github.com)... 140.82.112.5
Connecting to api.github.com (api.github.com)|140.82.112.5|:443... connected.
HTTP request sent, awaiting response... 502 Bad Gateway
2021-09-17 09:24:24 ERROR 502: Bad Gateway.

Notice: Array to string conversion in ...popular-package-analysis/download.php on line 61
wget failed: Array