check and log errors more often in deferred cleanup calls, and log remote-induced errors at lower priority

We normally check errors for all operations. But for some cleanup calls, eg
"defer file.Close()", we didn't. Now we also check and log most of those.
Partially because those errors can point to some mishandling or unexpected code
paths (eg file unexpected already closed). And in part to make it easier to use
"errcheck" to find the real missing error checks, there is too much noise now.

The log.Check function can now be used unconditionally for checking and logging
about errors. It adjusts the log level if the error is caused by a network
connection being closed, or a context is canceled or its deadline reached, or a
socket deadline is reached.

webapi/client.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"net/url"
 	"strings"
@@ -17,6 +18,7 @@ type Client struct {
 	Username   string // Added as HTTP basic authentication if not empty.
 	Password   string
 	HTTPClient *http.Client // Optional, defaults to http.DefaultClient.
+	Logger     *slog.Logger // Optional, defaults to slog.Default().
 }
 
 var _ Methods = Client{}
@@ -28,12 +30,24 @@ func (c Client) httpClient() *http.Client {
 	return http.DefaultClient
 }
 
+func (c Client) log(ctx context.Context) *slog.Logger {
+	if c.Logger != nil {
+		return c.Logger
+	}
+	return slog.Default()
+}
+
 func transact[T any](ctx context.Context, c Client, fn string, req any) (resp T, rerr error) {
 	hresp, err := httpDo(ctx, c, fn, req)
 	if err != nil {
 		return resp, err
 	}
-	defer hresp.Body.Close()
+	defer func() {
+		err := hresp.Body.Close()
+		if err != nil {
+			c.log(ctx).Debug("closing http response body", slog.Any("err", err))
+		}
+	}()
 
 	if hresp.StatusCode == http.StatusOK {
 		// Text and HTML of a message can each be 1MB. Another MB for other data would be a
@@ -52,7 +66,10 @@ func transactReadCloser(ctx context.Context, c Client, fn string, req any) (resp
 	body := hresp.Body
 	defer func() {
 		if body != nil {
-			body.Close()
+			err := body.Close()
+			if err != nil {
+				c.log(ctx).Debug("closing http response body", slog.Any("err", err))
+			}
 		}
 	}()
 	if hresp.StatusCode == http.StatusOK {