Fix mermaid error handling
Fix mermaid error handling

Yukaii/hackmd,jackycute/HackMD,Yukaii/hackmd,hackmdio/hackmd,PeterDaveHello/hackmd,PeterDaveHello/hackmd,Yukaii/hackmd,hackmdio/hackmd,jackycute/HackMD,jackycute/HackMD,PeterDaveHello/hackmd,hackmdio/hackmd