Check if files exists before trying to open them

[mountaindude]

Applies to both certificate files and Excel files (for importing master items).

Currently a missing file can cause output like below.
Better to just say "missing file ..." and exit.

➜  tools ./ctrl-q master-item-import --host 192.168.100.109 --app-id a3e0f5d2-000a-464f-998d-33d333b175d7 --auth-user-dir LAB --auth-user-id goran --auth-type cert --file-type excel --file ./ctrl-q-testdata.xlsx --sheet Sales --col-ref-by name --col-item-type "Master item type" --col-master-item-name "Master Item Name" --col-master-item-descr "Description" --col-master-item-label "Label" --col-master-item-expr "Expression" --col-master-item-tag "Tag"

2022-10-13T18:26:50.923Z info: Import master items from definitions in Excel file "./ctrl-q-testdata.xlsx"
2022-10-13T18:26:50.927Z error: Error: ENOENT: no such file or directory, open './ctrl-q-testdata.xlsx'
    at Object.openSync (node:fs:585:3)
    at Object.openSync (pkg/prelude/bootstrap.js:796:32)
    at Object.readFileSync (node:fs:453:35)
    at Object.readFileSync (pkg/prelude/bootstrap.js:1082:36)
    at read_binary (/snapshot/ctrl-q/node_modules/xlsx/xlsx.js:2730:44)
    at readSync (/snapshot/ctrl-q/node_modules/xlsx/xlsx.js:21877:69)
    at readFileSync (/snapshot/ctrl-q/node_modules/xlsx/xlsx.js:21909:9)
    at Object.parse (/snapshot/ctrl-q/node_modules/node-xlsx/lib/index.js:10:31)
    at importMasterItemFromExcel (/snapshot/ctrl-q/src/lib/importexcel.js:40:41)
    at importMasterItemFromFile (/snapshot/ctrl-q/src/lib/importexcel.js:324:9)
    at Command.<anonymous> (/snapshot/ctrl-q/src/ctrl-q.js:110:17)
    at Command.listener [as _actionHandler] (/snapshot/ctrl-q/node_modules/commander/lib/command.js:482:17)
    at /snapshot/ctrl-q/node_modules/commander/lib/command.js:1264:65
    at Command._chainOrCall (/snapshot/ctrl-q/node_modules/commander/lib/command.js:1158:12)
    at Command._parseCommand (/snapshot/ctrl-q/node_modules/commander/lib/command.js:1264:27)
    at /snapshot/ctrl-q/node_modules/commander/lib/command.js:1062:27