Add two checks not to crash when a file doesn't exist or can't be opened

[jmcarcell]

BEGINRELEASENOTES

• Add two checks not to crash when a file doesn't exist or can't be opened and when an empty list is passed to makeReader.

ENDRELEASENOTES

[tmadlener]

I suppose these would be caught by the actual readers, but we need them here as well, because we need to peek into the file?

[jmcarcell]

Yes, right now it crashes if the file doesn't exist because file is a nullptr. When there are N files only the first one is checked with this, the rest will be checked by the respective reader.

[tmadlener]

👍 Thanks.