Fix multiple source warning messages from error to warn

lib/bundler/cli/install.rb

@@ -202,15 +202,16 @@ def normalize_settings
     end
 
     def warn_ambiguous_gems
+      # TODO: remove this when we drop Bundler 1.x support
       Installer.ambiguous_gems.to_a.each do |name, installed_from_uri, *also_found_in_uris|
-        Bundler.ui.error "Warning: the gem '#{name}' was found in multiple sources."
-        Bundler.ui.error "Installed from: #{installed_from_uri}"
-        Bundler.ui.error "Also found in:"
-        also_found_in_uris.each {|uri| Bundler.ui.error "  * #{uri}" }
-        Bundler.ui.error "You should add a source requirement to restrict this gem to your preferred source."
-        Bundler.ui.error "For example:"
-        Bundler.ui.error "    gem '#{name}', :source => '#{installed_from_uri}'"
-        Bundler.ui.error "Then uninstall the gem '#{name}' (or delete all bundled gems) and then install again."
+        Bundler.ui.warn "Warning: the gem '#{name}' was found in multiple sources."
+        Bundler.ui.warn "Installed from: #{installed_from_uri}"
+        Bundler.ui.warn "Also found in:"
+        also_found_in_uris.each {|uri| Bundler.ui.warn "  * #{uri}" }
+        Bundler.ui.warn "You should add a source requirement to restrict this gem to your preferred source."
+        Bundler.ui.warn "For example:"
+        Bundler.ui.warn "    gem '#{name}', :source => '#{installed_from_uri}'"
+        Bundler.ui.warn "Then uninstall the gem '#{name}' (or delete all bundled gems) and then install again."
       end
     end
   end

lib/bundler/resolver.rb

@@ -414,7 +414,7 @@ def validate_resolved_specs!(resolved_specs)
         msg = msg.join("\n")
 
         raise SecurityError, msg if multisource_disabled
-        Bundler.ui.error "Warning: #{msg}"
+        Bundler.ui.warn "Warning: #{msg}"
       end
     end
   end