Catch CurlDownloadStrategyError in upgrade.rb.

This should resolve issue 18364 by using ofail to report the error
instead of raising CurlDownloadStrategyError directly. Since it's
a Runtime exception, this caused the process to exit if any downloads
errored out and any other packages to be skipped.  FormulaInstaller.build()
will skip any packages whose dependencies are not up to date, so this
shouldn't cause any dependency issues.