node crashes when promise rejection occurs in apiCallback

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version.

Issue Description

When trying to send a mail with wrong server configuration, the adapter returns a success but when the transporter promise is rejected it falls back to parse which makes node crash by throwing an error.

Steps to reproduce

Use bad configuration in the transported or simply return a rejected promise in apiCallback

Actual Outcome

Crash report printed from node_modules/parse-server/lib/ParseServer.js#L261

Expected Outcome

The error should be handled or we should have the possibility to wait for a sucess/failure of the sending before returning to the client.

Failing Test Case / Pull Request

Environment

• API Mail Adapter version: 2.2.0
• Parse Server version: 5.4.0

Logs

express:router dispatching POST /requestPasswordReset
node_modules/parse-server/lib/ParseServer.js:261
          throw err;
          ^
Waiting for the debugger to disconnect...

Start at node_modules/parse-server/lib/ParseServer.js#L261 and trace how a rejected transporter promise or apiCallback rejection reaches that throw. Reproduce the issue with bad mail-server configuration or a rejected apiCallback promise. Done should prevent the Node.js process from crashing and define how the sending failure is reported, but the expected client-facing behavior remains unspecified.