Fetching: log more exceptions (#32736)

Include exception info related to url retrieval in debug messages
which otherwise would be swallowed. This is intended to be useful
for detecting if CA configuration interferes with downloads from
HTTPS links.

lib/spack/spack/fetch_strategy.py

@@ -338,6 +338,7 @@ def fetch(self):
         errors = []
         for url in self.candidate_urls:
             if not web_util.url_exists(url, self.curl):
+                tty.debug("URL does not exist: " + url)
                 continue
 
             try:

lib/spack/spack/util/web.py

@@ -444,7 +444,8 @@ def url_exists(url, curl=None):
     try:
         read_from_url(url)
         return True
-    except (SpackWebError, URLError):
+    except (SpackWebError, URLError) as e:
+        tty.debug("Failure reading URL: " + str(e))
         return False