Support custom messages for failed expectations

Fixes https://gitlab.com/arctic-fox/spectator/-/issues/28

spec/custom_message_spec.cr

@@ -0,0 +1,31 @@
+require "./spec_helper"
+
+Spectator.describe Spectator do
+  it "supports custom expectation messages" do
+    expect do
+      expect(false).to be_true, "paradox!"
+    end.to raise_error(Spectator::ExampleFailed, "paradox!")
+  end
+
+  it "supports custom expectation messages with a proc" do
+    count = 0
+    expect do
+      expect(false).to be_true, ->{ count += 1; "Failed #{count} times" }
+    end.to raise_error(Spectator::ExampleFailed, "Failed 1 times")
+  end
+
+  context "not_to" do
+    it "supports custom expectation messages" do
+      expect do
+        expect(true).not_to be_true, "paradox!"
+      end.to raise_error(Spectator::ExampleFailed, "paradox!")
+    end
+
+    it "supports custom expectation messages with a proc" do
+      count = 0
+      expect do
+        expect(true).not_to be_true, ->{ count += 1; "Failed #{count} times" }
+      end.to raise_error(Spectator::ExampleFailed, "Failed 1 times")
+    end
+  end
+end