Raise ArgumentError when trying to resolve an unknown adapter URI

If we were to fall through, undefined schemes would always end up as a
file adapter. This is generally undesired since it hides the fact that
we have not found a suitable adapter. Most of the time, this will be a
configuration error which should be reported early.

If a user still wants to create a file adapter with a filename that
looks like a URI, they can create a Rackstash::Adapter::File object
manually.

lib/rackstash/adapter.rb

@@ -110,7 +110,9 @@ module Rackstash
         scheme = uri.scheme || uri.opaque
 
         return unless scheme
-        adapter_class = adapter_schemes.fetch(scheme.to_s.downcase) { return }
+        adapter_class = adapter_schemes.fetch(scheme.to_s.downcase) {
+          raise ArgumentError, "No log adapter found for URI #{uri}"
+        }
 
         if adapter_class.respond_to?(:from_uri)
           adapter_class.from_uri(uri)

spec/rackstash/adapter_spec.rb

@@ -261,8 +261,9 @@ describe Rackstash::Adapter do
           unknown_uri = 'unknown://example.com'
 
           expect(adapter).to_not receive(:from_uri)
-          expect(adapter).to receive(:new).with(unknown_uri).and_call_original
-          expect(described_class[unknown_uri]).to be_an Rackstash::Adapter::BaseAdapter
+          expect(adapter).not_to receive(:new)
+          expect { described_class[unknown_uri] }
+            .to raise_error ArgumentError, "No log adapter found for URI unknown://example.com"
         end
       end
     end