Hopefully catch all errors during launch process.

app/assets/js/scripts/landing.js

@@ -103,8 +103,8 @@ function setLaunchEnabled(val){
 // Bind launch button
 document.getElementById('launch_button').addEventListener('click', async e => {
     loggerLanding.info('Launching game..')
+    try {
         const server = (await DistroAPI.getDistribution()).getServerById(ConfigManager.getSelectedServer())
-    const mcVersion = server.rawServer.minecraftVersion
         const jExe = ConfigManager.getJavaExecutable(ConfigManager.getSelectedServer())
         if(jExe == null){
             await asyncSystemScan(server.effectiveJavaOptions)
@@ -118,10 +118,15 @@ document.getElementById('launch_button').addEventListener('click', async e => {
             if(details != null){
                 loggerLanding.info('Jvm Details', details)
                 await dlAsync()
+
             } else {
                 await asyncSystemScan(server.effectiveJavaOptions)
             }
         }
+    } catch(err) {
+        loggerLanding.error('Unhandled error in during launch process.', err)
+        showLaunchFailure('Error During Launch', 'See console (CTRL + Shift + i) for more details.')
+    }
 })
 
 // Bind settings button
@@ -319,9 +324,14 @@ async function asyncSystemScan(effectiveJavaOptions, launchAfter = true){
         )
         setOverlayHandler(() => {
             setLaunchDetails('Preparing Java Download..')
-            
-            downloadJava(effectiveJavaOptions, launchAfter)
             toggleOverlay(false)
+            
+            try {
+                downloadJava(effectiveJavaOptions, launchAfter)
+            } catch(err) {
+                loggerLanding.error('Unhandled error in Java Download', err)
+                showLaunchFailure('Error During Java Download', 'See console (CTRL + Shift + i) for more details.')
+            }
         })
         setDismissHandler(() => {
             $('#overlayContent').fadeOut(250, () => {
@@ -374,6 +384,10 @@ async function downloadJava(effectiveJavaOptions, launchAfter = true) {
         ConfigManager.getDataDirectory(),
         effectiveJavaOptions.distribution)
 
+    if(asset == null) {
+        throw new Error('Failed to find OpenJDK distribution.')
+    }
+
     let received = 0
     await downloadFile(asset.url, asset.path, ({ transferred }) => {
         received = transferred
@@ -385,9 +399,8 @@ async function downloadJava(effectiveJavaOptions, launchAfter = true) {
         loggerLanding.warn(`Java Download: Expected ${asset.size} bytes but received ${received}`)
         if(!await validateLocalFile(asset.path, asset.algo, asset.hash)) {
             log.error(`Hashes do not match, ${asset.id} may be corrupted.`)
-
+            // Don't know how this could happen, but report it.
-            // TODO Make error handling graceful.
+            throw new Error('Downloaded JDK has bad hash, file may be corrupted.')
-            throw new Error('JDK download had problems')
         }
     }
 
@@ -492,18 +505,32 @@ async function dlAsync(login = true) {
 
     loggerLaunchSuite.info('Validating files.')
     setLaunchDetails('Validating file integrity..')
-    const invalidFileCount = await fullRepairModule.verifyFiles(percent => {
+    let invalidFileCount = 0
+    try {
+        invalidFileCount = await fullRepairModule.verifyFiles(percent => {
             setLaunchPercentage(percent)
         })
         setLaunchPercentage(100)
+    } catch (err) {
+        loggerLaunchSuite.error('Error during file validation.')
+        showLaunchFailure('Error During File Verification', err.displayable || 'See console (CTRL + Shift + i) for more details.')
+        return
+    }
+    
 
     if(invalidFileCount > 0) {
         loggerLaunchSuite.info('Downloading files.')
         setLaunchDetails('Downloading files..')
+        try {
             await fullRepairModule.download(percent => {
                 setDownloadPercentage(percent)
             })
             setDownloadPercentage(100)
+        } catch(err) {
+            loggerLaunchSuite.error('Error during file download.')
+            showLaunchFailure('Error During File Download', err.displayable || 'See console (CTRL + Shift + i) for more details.')
+            return
+        }
     } else {
         loggerLaunchSuite.info('No invalid files, skipping download.')
     }