Raise MissingAssetError in compute_asset_path instead of each resolver

Follow-up to 0fd7814:
Rather than adding this `raise` to each resolver's `resolve` method,
let's move it into Propshaft's `compute_asset_path` implementation.
`compute_asset_path` is the ActionView method that asset pipeline gems
are expected to implement [1] [2], which makes it 1) the highest level
for us to catch missing assets and 2) the entry point for anyone looking
to understand how Propshaft integrates with the ActionView helpers
(thus, it's helpful to have that method's definition explicitly include
the exception it raises, just like sprockets-rails does [3]).

[1]: https://github.com/rails/rails/blob/099289b6360ac82d1e0fa0a5592ac10cfc05e6e0/actionview/lib/action_view/helpers/asset_url_helper.rb#L132-L133
[2]: https://github.com/rails/rails/blob/099289b6360ac82d1e0fa0a5592ac10cfc05e6e0/actionview/lib/action_view/helpers/asset_url_helper.rb#L262-L265
[3]: https://github.com/rails/sprockets-rails/blob/118ce60b1ffeb7a85640661b014cd2ee3c4e3e56/lib/sprockets/rails/helper.rb#L84

lib/propshaft/helper.rb

@@ -1,5 +1,5 @@
 module Propshaft::Helper
   def compute_asset_path(path, options = {})
-    Rails.application.assets.resolver.resolve(path)
+    Rails.application.assets.resolver.resolve(path) || raise(Propshaft::MissingAssetError.new(path))
   end
 end

lib/propshaft/resolver/dynamic.rb

@@ -9,8 +9,6 @@ def initialize(load_path:, prefix:)
     def resolve(logical_path)
       if asset = load_path.find(logical_path)
         File.join prefix, asset.digested_path
-      else
-        raise Propshaft::MissingAssetError.new(logical_path)
       end
     end
   end

lib/propshaft/resolver/static.rb

@@ -9,8 +9,6 @@ def initialize(manifest_path:, prefix:)
     def resolve(logical_path)
       if asset_path = parsed_manifest[logical_path]
         File.join prefix, asset_path
-      else
-        raise Propshaft::MissingAssetError.new(logical_path)
       end
     end
 

test/propshaft/resolver/dynamic_test.rb

@@ -12,9 +12,7 @@ class Propshaft::Resolver::DynamicTest < ActiveSupport::TestCase
       @resolver.resolve("one.txt")
   end
 
-  test "resolving missing asset raises a custom error" do
-    assert_raise Propshaft::MissingAssetError do
-      assert_nil @resolver.resolve("nowhere.txt")
-    end
+  test "resolving missing asset returns nil" do
+    assert_nil @resolver.resolve("nowhere.txt")
   end
 end

test/propshaft/resolver/static_test.rb

@@ -15,9 +15,7 @@ class Propshaft::Resolver::StaticTest < ActiveSupport::TestCase
       @resolver.resolve("one.txt")
   end
 
-  test "resolving missing asset raises a custom error" do
-    assert_raise Propshaft::MissingAssetError do
-      assert_nil @resolver.resolve("nowhere.txt")
-    end
+  test "resolving missing asset returns nil" do
+    assert_nil @resolver.resolve("nowhere.txt")
   end
 end