catch install-time errors on windows

[jaimergp]

Description

Closes #544

Closes #551

In #509 I forgot to add one piece of the conda packages extraction logic. This resulted in failed installs. Readding this piece fixes #544.

However, this error wasn't caught by the CI pipeline because of #551!

Note that the silent mode of NSIS installers (/S) does not print anything to STDOUT. Additionally, most of the dialogs are configured to "ignore" the warning if an error occurred, thus moving on. The exit code does not change with errors! We can't rely on that to detect whether something failed. Instead, the solution was:

• The CI now uses a special NSIS build with log-dumping abilities (normal NSIS does not have this feature). I want to add *log* variants to the nsis package in conda-forge/defaults, but for now we are manually patching.
• Errors are printed to the log file, even if ignored
• conda install ... step will throw an error too, if needed (it didn't before!)
• The CI will inspect the log for :error: strings planted by our NSI script
• If there were any errors, these are printed and reported the installer is reported as failing.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Finally, we have eyes on Windows :D

This is ready for review now.

[jaimergp]

Note Windows are now failing, as reported in #544

We want to merge with failures. I'll debug 544 in a separate PR.

Edit: looks like the fix is simpler! I had forgotten a block in the NSI template.

[larsoner]

LGTM +1 for merge

[hoechenberger]

Thanks @jaimergp!