bugfix: fetch prefers to fetch local mirrors over remote resources (#13545)

- [x] insert at beginning of list so fetch grabs local mirrors before remote resources
- [x] update the S3FetchStrategy so that it throws a SpackError if the fetch fails.  
      Before, it was throwing URLError, which was not being caught in stage.py.
- [x] move error handling out of S3FetchStrategy and into web_util.read_from_url()
- [x] pass string instead of URLError to SpackWebError

lib/spack/spack/fetch_strategy.py

@@ -1129,7 +1129,7 @@ def fetch(self):
 
         parsed_url = url_util.parse(self.url)
         if parsed_url.scheme != 's3':
-            raise ValueError(
+            raise FetchError(
                 'S3FetchStrategy can only fetch from s3:// urls.')
 
         tty.msg("Fetching %s" % self.url)
@@ -1395,7 +1395,7 @@ class NoCacheError(FetchError):
 
 
 class FailedDownloadError(FetchError):
-    """Raised wen a download fails."""
+    """Raised when a download fails."""
     def __init__(self, url, msg=""):
         super(FailedDownloadError, self).__init__(
             "Failed to fetch file from URL: %s" % url, msg)

lib/spack/spack/stage.py

@@ -433,11 +433,9 @@ def fetch(self, mirror_only=False):
 
             # Add URL strategies for all the mirrors with the digest
             for url in urls:
-                fetchers.append(fs.from_url_scheme(
-                    url, digest, expand=expand, extension=extension))
-                # fetchers.insert(
-                #     0, fs.URLFetchStrategy(
-                #         url, digest, expand=expand, extension=extension))
+                fetchers.insert(
+                    0, fs.from_url_scheme(
+                        url, digest, expand=expand, extension=extension))
 
             if self.default_fetcher.cachable:
                 for rel_path in reversed(list(self.mirror_paths)):

lib/spack/spack/util/web.py

@@ -177,7 +177,12 @@ def read_from_url(url, accept_content_type=None):
 
     # Do the real GET request when we know it's just HTML.
     req.get_method = lambda: "GET"
-    response = _urlopen(req, timeout=_timeout, context=context)
+
+    try:
+        response = _urlopen(req, timeout=_timeout, context=context)
+    except URLError as err:
+        raise SpackWebError('Download failed: {ERROR}'.format(
+            ERROR=str(err)))
 
     if accept_content_type and not is_web_url:
         content_type = response.headers.get('Content-type')