proxy_pass: more descriptive error messages

This should make it easier to figure out where certain
errors are coming from and perhaps fix problems with
upstreams, too.

This helped me track down the problem causing public-inbox WWW
component running under Perl v5.20.2 on my Debian jessie system
to break and drop connections going through
Plack::Middleware::Deflater with gzip:

  https://public-inbox.org/meta/20160607071401.29325-1-e@80x24.org/

Perl 5.14.2 on Debian wheezy did not detect this problem :x

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/yahns/req_res.rb b/lib/yahns/req_res.rb
index 9bb8f35..041b908 100644
--- a/lib/yahns/req_res.rb
+++ b/lib/yahns/req_res.rb
@@ -42,7 +42,8 @@ class Yahns::ReqRes < Kgio::Socket # :nodoc:
           # continue looping in middle "case @resbuf" loop
         when :wait_readable
           return rv # spurious wakeup
-        when nil then return c.proxy_err_response(502, self, nil)
+        when nil
+          return c.proxy_err_response(502, self, 'upstream EOF (headers)')
         end # NOT looping here
 
       when String # continue reading trickled response headers from upstream
@@ -50,7 +51,8 @@ class Yahns::ReqRes < Kgio::Socket # :nodoc:
         case rv = kgio_tryread(0x2000, buf)
         when String then res = req.headers(@hdr, resbuf << rv) and break
         when :wait_readable then return rv
-        when nil then return c.proxy_err_response(502, self, nil)
+        when nil
+          return c.proxy_err_response(502, self, 'upstream EOF (big headers)')
         end while true
         @resbuf = false