Prosody Appimage: supports only x86_64:

* stop trying to generated an ARM64 AppImage (as it is buggy)
* display a warning in the settings if incompatible CPU detected
* documentation for a workaround

Implements #120.

server/lib/prosody/ctl.ts

@@ -100,6 +100,10 @@ async function prosodyCtl (options: RegisterServerOptions, command: string): Pro
     throw new Error(`Invalid prosodyctl command '${command}'`)
   }
   return new Promise((resolve, reject) => {
+    if (!filePaths.execCtl) {
+      reject(new Error('Missing prosodyctl command executable'))
+      return
+    }
     let d: string = ''
     let e: string = ''
     let m: string = ''
@@ -247,6 +251,11 @@ async function ensureProsodyRunning (options: RegisterServerOptions): Promise<vo
 
   const filePaths = config.paths
 
+  if (!filePaths.exec) {
+    logger.info('No Prosody executable, cant run.')
+    return
+  }
+
   // launch prosody
   const execCmd = filePaths.exec + (filePaths.execArgs.length ? ' ' + filePaths.execArgs.join(' ') : '')
   logger.info('Going to launch prosody (' + execCmd + ')')