Allow the ZAP UI to load when a custom xml file is not found instead …

…of throwing an error (project-chip#1225)

- The changes to import-json.js allows the ZAP UI to load instead of throwing an error and locking ZAP.
- Now the error message appears in the ZAP notification's pane so that the user can see a detailed error and act upon it
- Minor cleanup
- JIRA: ZAPP-1148

src-electron/importexport/import-json.js

@@ -136,7 +136,8 @@ async function importSinglePackage(db, pkg, zapFilePath, packageMatch) {
       env.logDebug(`No packages of type ${pkg.type} found in the database.`)
       return null
     } else {
-      throw new Error(`No packages of type ${pkg.type} found in the database.`)
+      env.logError(`No packages of type ${pkg.type} found in the database.`)
+      return null
     }
   } else if (packages.length == 1) {
     env.logDebug(

src-electron/zcl/zcl-loader-silabs.js

@@ -2220,7 +2220,7 @@ async function loadIndividualSilabsFile(db, filePath, sessionId) {
     querySessionNotification.setNotification(
       db,
       'ERROR',
-      `Error reading xml file: ${filePath}\n` + err.message,
+      `Error reading xml file: ${filePath}, Error Message: ` + err.message,
       sessionId,
       1,
       0