Make SSHKit::Runner::Parallel fail slow

Using a different SSHKit runner doesn't work well, because the group
runner uses the Parallel runner internally. So instead we'll patch its
behaviour to fail slow.

We'll also get it to return all the errors so we can report on all the
hosts that failed.

bin/kamal

@@ -7,6 +7,15 @@ require "kamal"
 
 begin
   Kamal::Cli::Main.start(ARGV)
+rescue SSHKit::Runner::MultipleExecuteError => e
+  e.execute_errors.each do |execute_error|
+    puts "  \e[31mERROR (#{execute_error.cause.class}): #{execute_error.message}\e[0m"
+  end
+  if ENV["VERBOSE"]
+    puts "Backtrace for the first error:"
+    puts e.execute_errors.first.cause.backtrace
+  end
+  exit 1
 rescue SSHKit::Runner::ExecuteError => e
   puts "  \e[31mERROR (#{e.cause.class}): #{e.message}\e[0m"
   puts e.cause.backtrace if ENV["VERBOSE"]