Add additional error checking for createMergedFile method

[jerboa88]

What feature would you like?
Add additional error checking to the createMergedFile method of PDFManager to help avoid unexpected bugs. Since this method is only called once we can afford to do extra checks. For example:

• Check that at least 1 PDF file has been added
• Check that the size of the fileIds array matches the size of the file map
• Add a separate try/catch block for setting metadata so that we can soft-fail and return a warning to the user instead of cancelling the merge altogether when there is an error
• Figure out if it is possible to return warnings generated by pdf-lib. If some of the input files are only partially corrupted, it is still possible to merge them atm. However, pdf-lib displays warnings in the console when it encounters parsing issues so it would be good to inform the user that there were issues with the input files