Better error messaging for curl failures

A customer recently had an issue with curl failing to download `fetch`. The error is:

```
Step 10/24 : RUN curl -Ls https://raw.githubusercontent.com/gruntwork-io/gruntwork-installer/master/bootstrap-gruntwork-installer.sh | bash /dev/stdin --version v0.0.23
---> Running in b606ba274890
Installing gruntwork-install...
Installing fetch version v0.3.2 to /usr/local/bin/fetch
Downloading https://github.com/gruntwork-io/fetch/releases/download/v0.3.2/fetch_linux_amd64 to /tmp/gruntwork-bootstrap-download-JAaGkA
ERROR: Expected status code 200 but got 302 when downloading https://github.com/gruntwork-io/fetch/releases/download/v0.3.2/fetch_linux_amd64
The command '/bin/sh -c curl -Ls https://raw.githubusercontent.com/gruntwork-io/gruntwork-installer/master/bootstrap-gruntwork-installer.sh | bash /dev/stdin --version v0.0.23' returned a non-zero code: 1
```

Upon investigation, the error turned out to be a network monitoring proxy that was in the middle of the SSL connection causing curl to fail SSL validation. This was being swallowed silently.