diff --git a/src/pip/_internal/cli/progress_bars.py b/src/pip/_internal/cli/progress_bars.py
index d90ae67cd73..f357b89fef4 100644
--- a/src/pip/_internal/cli/progress_bars.py
+++ b/src/pip/_internal/cli/progress_bars.py
@@ -14,7 +14,7 @@
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
-    from typing import Any, Dict, List
+    from typing import Any, Dict, List, Optional
 
 try:
     from pip._vendor import colorama
@@ -24,6 +24,18 @@
     colorama = None
 
 
+def _signal_unless_backgrounded(signum, handler):
+    # type: (int, Any) -> Optional[Any]
+    try:
+        return signal(signum, handler)
+    except ValueError:
+        # FIXME: this otherwise doesn't work when called from a non-main
+        # thread. This therefore fails if we try to download more than one
+        # wheel at once via threading, which calls back to Downloader, which
+        # uses this progress bar.
+        return None
+
+
 def _select_progress_class(preferred, fallback):
     # type: (Bar, Bar) -> Bar
     encoding = getattr(preferred.file, "encoding", None)
@@ -84,11 +96,8 @@ def __init__(self, *args, **kwargs):
             **kwargs
         )
 
-        # FIXME: this otherwise doesn't work when called from a non-main
-        # thread. This therfore fails if we try to download more than one wheel
-        # at once.
-        # self.original_handler = signal(SIGINT, self.handle_sigint)
-        self.original_handler = None
+        self.original_handler = _signal_unless_backgrounded(
+            SIGINT, self.handle_sigint)
 
         # If signal() returns None, the previous handler was not installed from
         # Python, and we cannot restore it. This probably should not happen,
@@ -107,7 +116,7 @@ def finish(self):
         normally, or gets interrupted.
         """
         super(InterruptibleMixin, self).finish()  # type: ignore
-        signal(SIGINT, self.original_handler)
+        _signal_unless_backgrounded(SIGINT, self.original_handler)
 
     def handle_sigint(self, signum, frame):  # type: ignore
         """
diff --git a/src/pip/_internal/req/req_set.py b/src/pip/_internal/req/req_set.py
index 6b819c0aa4c..4b959001492 100644
--- a/src/pip/_internal/req/req_set.py
+++ b/src/pip/_internal/req/req_set.py
@@ -7,7 +7,8 @@
 from pip._vendor.packaging.utils import canonicalize_name
 
 from pip._internal.distributions.shallow_wheel import (
-    DistributionNeedingFinalHydration)
+    DistributionNeedingFinalHydration,
+)
 from pip._internal.exceptions import InstallationError
 from pip._internal.models.wheel import Wheel
 from pip._internal.utils import compatibility_tags
@@ -66,8 +67,14 @@ def perform_all_final_hydration(self):
         if not self.dists_needing_final_hydration:
             return
 
+        exceptions = []
         def do_hydrate(dist):
-            dist.finally_hydrate()
+            # type: (DistributionNeedingFinalHydration) -> None
+            try:
+                dist.finally_hydrate()
+            except Exception as e:
+                exceptions.append(e)
+
         all_threads = [
             threading.Thread(
                 target=do_hydrate, name='download dist {}'.format(dist),
@@ -78,6 +85,9 @@ def do_hydrate(dist):
             t.start()
         for t in all_threads:
             t.join()
+        if exceptions:
+            raise ValueError('at least one thread failed (errors below):\n{}'
+                             .format('\n'.join(str(e) for e in exceptions)))
 
     def add_unnamed_requirement(self, install_req):
         # type: (InstallRequirement) -> None
diff --git a/src/pip/_internal/resolution/resolvelib/resolver.py b/src/pip/_internal/resolution/resolvelib/resolver.py
index 623f47f36b1..5c9ad286105 100644
--- a/src/pip/_internal/resolution/resolvelib/resolver.py
+++ b/src/pip/_internal/resolution/resolvelib/resolver.py
@@ -7,7 +7,8 @@
 from pip._vendor.resolvelib import Resolver as RLResolver
 
 from pip._internal.distributions.shallow_wheel import (
-    DistributionNeedingFinalHydration)
+    DistributionNeedingFinalHydration,
+)
 from pip._internal.exceptions import DistributionNotFound, InstallationError
 from pip._internal.req.req_set import RequirementSet
 from pip._internal.resolution.base import BaseResolver