Return every error as {"error": "..."}

Echo errors, middleware and handlers used three different formats (message object, error object, bare string). One error handler now writes all of them the same way; status codes are unchanged.

internal/server/errors.go

@@ -0,0 +1,47 @@
+package server
+
+import (
+	"errors"
+	"net/http"
+
+	"music-server/internal/logging"
+
+	"github.com/labstack/echo/v5"
+	"go.uber.org/zap"
+)
+
+// ErrorResponse is the body of every error response.
+type ErrorResponse struct {
+	Error string `json:"error" example:"Not Found"`
+}
+
+// httpErrorHandler writes every error as an ErrorResponse.
+func httpErrorHandler(c *echo.Context, err error) {
+	if r, _ := echo.UnwrapResponse(c.Response()); r != nil && r.Committed {
+		return
+	}
+
+	code := http.StatusInternalServerError
+	message := http.StatusText(code)
+	var he *echo.HTTPError
+	var sc echo.HTTPStatusCoder
+	switch {
+	case errors.As(err, &he):
+		code = he.Code
+		message = he.Message
+	case errors.As(err, &sc):
+		code = sc.StatusCode()
+	}
+	if code >= http.StatusInternalServerError && he == nil {
+		logging.GetLogger().Error("Unhandled error", zap.String("path", c.Request().URL.Path), zap.Error(err))
+	}
+	if message == "" {
+		message = http.StatusText(code)
+	}
+
+	if c.Request().Method == http.MethodHead {
+		_ = c.NoContent(code)
+		return
+	}
+	_ = c.JSON(code, ErrorResponse{Error: message})
+}