mark recoverability of DNS erros correctly

This changes HTTP client to not retry on certain DNS errors. From the [list of possible DNS errors](https://github.com/JuliaLang/julia/blob/ec8df3da3597d0acd503ff85ac84a5f8f73f625b/stdlib/Sockets/src/addrinfo.jl#L108-L112) only EAI_AGAIN may be recoverable. This also changes the error code comparison to use `UV_EAI_AGAIN`, because that is what a [DNSError instance would contain](https://github.com/JuliaLang/julia/blob/ec8df3da3597d0acd503ff85ac84a5f8f73f625b/stdlib/Sockets/src/addrinfo.jl#L108-L112).

src/clientlayers/RetryRequest.jl

@@ -17,7 +17,7 @@ increasing delay is introduced between attempts to avoid exacerbating network
 congestion.
 
 By default, requests that have a retryable body, where the request wasn't written
-or is idempotent will be retries. If the request is made and a response is received
+or is idempotent will be retried. If the request is made and a response is received
 with a status code of 403, 408, 409, 429, or 5xx, the request will be retried.
 
 `retries` controls the # of total retries that will be attempted.
@@ -76,10 +76,17 @@ function retrylayer(handler)
     end
 end
 
-const EAI_AGAIN = 2
 isrecoverable(ex) = true
 isrecoverable(ex::CapturedException) = isrecoverable(ex.ex)
-isrecoverable(ex::ConnectError) = ex.error isa Sockets.DNSError && ex.error.code == EAI_AGAIN ? false : true
+# Treat all DNS errors except `EAI_AGAIN`` as non-recoverable
+# Ref: https://github.com/JuliaLang/julia/blob/ec8df3da3597d0acd503ff85ac84a5f8f73f625b/stdlib/Sockets/src/addrinfo.jl#L108-L112
+function isrecoverable(ex::ConnectError)
+    if ex.error isa Sockets.DNSError
+        return (ex.error.code == Base.UV_EAI_AGAIN) ? true : false
+    else
+        return true
+    end
+end
 isrecoverable(ex::StatusError) = retryable(ex.status)
 
 function _retry_check(s, ex, req, check)