DEV: Add a try step to services

This patch adds a new step to services named `try`.

It’s useful to rescue exceptions that some steps could raise. That way,
if an exception is caught, the service will stop its execution and can
be inspected like with any other steps.

Just wrap the steps that can raise with a `try` block:
```ruby
try do
  step :step_that_can_raise
  step :another_step_that_can_raise
end
```
By default, `try` will catch any exception inheriting from
`StandardError`, but we can specify what exceptions to catch:
```ruby
try(ArgumentError, RuntimeError) do
  step :will_raise
end
```

An outcome matcher has been added: `on_exceptions`. By default it will
be executed for any exception caught by the `try` step.
Here also, we can specify what exceptions to catch:
```ruby
on_exceptions(ArgumentError, RuntimeError) do |exception|
  …
end
```

Finally, an RSpec matcher has been added:
```ruby
  it { is_expected.to fail_with_exception }
  # or
  it { is_expected.to fail_with_exception(ArgumentError) }
```

spec/lib/service/runner_spec.rb

@@ -147,6 +147,26 @@ RSpec.describe Service::Runner do
     end
   end
 
+  class FailureTryService
+    include Service::Base
+
+    try { step :raising_step }
+
+    def raising_step
+      raise "BOOM"
+    end
+  end
+
+  class SuccessTryService
+    include Service::Base
+
+    try { step :non_raising_step }
+
+    def non_raising_step
+      true
+    end
+  end
+
   describe ".call" do
     subject(:runner) { described_class.call(service, dependencies, &actions_block) }
 
@@ -429,6 +449,30 @@ RSpec.describe Service::Runner do
       end
     end
 
+    context "when using the on_exceptions action" do
+      let(:actions) { <<-BLOCK }
+        proc do |result|
+          on_exceptions { |exception| exception.message == "BOOM" }
+        end
+      BLOCK
+
+      context "when the service fails" do
+        let(:service) { FailureTryService }
+
+        it "runs the provided block" do
+          expect(runner).to be true
+        end
+      end
+
+      context "when the service does not fail" do
+        let(:service) { SuccessTryService }
+
+        it "does not run the provided block" do
+          expect(runner).not_to eq true
+        end
+      end
+    end
+
     context "when using several actions together" do
       let(:service) { FailureService }
       let(:actions) { <<-BLOCK }