Fix libyaml download failure rescue under miniruby

[XrXr]

I tried to build Ruby on a system without libyaml today and realized
that my attempt from #557 doesn't
fix the error in #552. I still got
the same LoadError from digest.

Since LoadError is not a StandardError, a plain rescue doesn't catch
it. Catch LoadError explicitly instead and reduce the scope of the
begin block.

I tested this change in a Ruby build on macOS without libyaml installed
and confirmed that make continues with a warning instead of aborting:

*** Following extensions are not compiled:
psych:
        Could not be configured. It will not be installed.
        ...

[XrXr]

For only graceful failures on the Ruby side, we could apply:

diff --git a/ext/extmk.rb b/ext/extmk.rb
index a440af27fc..fd6518fa6e 100755
--- a/ext/extmk.rb
+++ b/ext/extmk.rb
@@ -224,7 +224,7 @@ def extmake(target, basedir = 'ext', maybestatic = true)
        end
       rescue SystemExit
        # ignore
-      rescue => error
+      rescue StandardError, ScriptError => error
         ok = false
       ensure
        rm_f "conftest*"

This PR gives better error message in addition to graceful failures. We may want to change extmk.rb for similar issues in the future, though.

[XrXr]

I'm going to merge this since it should fix https://bugs.ruby-lang.org/issues/18790 for people that don't have libyaml installed on their system. Feel free to revert/ make changes if you notice something off.