Provide better error message for .md files not containing valid Markdown

[ghtyrant]

Is your feature request related to a problem? Please describe.
I just tried mmdc for the first time, copy-pasted a sequence diagram example from the mermaid documentation and out of habit named my input file test.md.
Running mmdc as follows:
mmdc -i test.md -o test.svg -t dark -b transparent
yields the following output

/usr/lib/node_modules/@mermaid-js/mermaid-cli/index.bundle.js:210
    if (matches.length > 0) {
                ^

TypeError: Cannot read properties of null (reading 'length')
    at /usr/lib/node_modules/@mermaid-js/mermaid-cli/index.bundle.js:210:17
    at Generator.next (<anonymous>)
    at step (/usr/lib/node_modules/@mermaid-js/mermaid-cli/index.bundle.js:4:191)
    at /usr/lib/node_modules/@mermaid-js/mermaid-cli/index.bundle.js:4:361

Describe the solution you'd like
A more descriptive error message, e.g. "Input file expected to contain valid Markdown. Don't use the file extension .md if you just want to render a mermaid diagram." or probably something less clunky would be helpful.

[MindaugasLaganeckas]

@ghtyrant : Thanks for reporting the issue! Would you like to work on the solution? I would be glad to review your PR 😄

[ghtyrant]

@MindaugasLaganeckas Done 🙂