cli/parser: Better error message for cask on linux

1. Validate options before constraint violations. This allows
us to error out when --cask is passed on Linux before getting
a constraint violation when --cask and --formula are set.
2. Skip printing the help page when --cask is passed on Linux.

Library/Homebrew/cli/parser.rb

@@ -332,8 +332,8 @@ module Homebrew
         set_default_options
 
         unless ignore_invalid_options
-          check_constraint_violations
           validate_options
+          check_constraint_violations
           check_named_args(named_args)
         end
 

Library/Homebrew/extend/os/linux/parser.rb

@@ -15,7 +15,9 @@ module Homebrew
         return unless @args.respond_to?(:cask?)
         return unless @args.cask?
 
-        raise UsageError, "Casks are not supported on Linux"
+        # NOTE: We don't raise a UsageError here because
+        # we don't want to print the help page.
+        raise "Invalid usage: Casks are not supported on Linux"
       end
     end
   end

Library/Homebrew/test/cli/parser_spec.rb

@@ -564,6 +564,7 @@ describe Homebrew::CLI::Parser do
   end
 
   describe "--cask on linux", :needs_linux do
+    context "without --formula switch" do
       subject(:parser) do
         described_class.new do
           switch "--cask"
@@ -571,7 +572,29 @@ describe Homebrew::CLI::Parser do
       end
 
       it "throws an error when defined" do
-      expect { parser.parse(["--cask"]) }.to raise_error UsageError, /Casks are not supported on Linux/
+        expect { parser.parse(["--cask"]) }
+          .to raise_error RuntimeError, "Invalid usage: Casks are not supported on Linux"
+      end
+    end
+
+    context "with conflicting --formula switch" do
+      subject(:parser) do
+        described_class.new do
+          switch "--cask"
+          switch "--formula"
+          conflicts "--cask", "--formula"
+        end
+      end
+
+      it "throws an error when --cask defined" do
+        expect { parser.parse(["--cask"]) }
+          .to raise_error RuntimeError, "Invalid usage: Casks are not supported on Linux"
+      end
+
+      it "throws an error when both defined" do
+        expect { parser.parse(["--cask", "--formula"]) }
+          .to raise_error RuntimeError, "Invalid usage: Casks are not supported on Linux"
+      end
     end
   end