[MM-56792] Clarify large plugin upload error (#26271)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/api4/plugin.go

@@ -20,7 +20,8 @@ import (
 )
 
 const (
-	MaximumPluginFileSize = 50 * 1024 * 1024
+	// MaxPluginMemory is the maximum number of bytes to hold in memory when reading a plugin bundle.
+	MaxPluginMemory = 50 * 1024 * 1024
 )
 
 func (api *API) InitPlugin() {
@@ -57,7 +58,11 @@ func uploadPlugin(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := r.ParseMultipartForm(MaximumPluginFileSize); err != nil {
+	if err := r.ParseMultipartForm(MaxPluginMemory); err != nil {
+		if err.Error() == "http: request body too large" {
+			c.Err = model.NewAppError("uploadPlugin", "api.plugin.upload.file_too_large.app_error", nil, "", http.StatusRequestEntityTooLarge)
+			return
+		}
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}