signing: fixes for bootimg hdr_v1 and hdr_v2

- increase SignBoot bootimg header version maximum from 4 to 8 (upstream AOSP is already at 3) and make a variable for future ease
- hdr read size of 1024 bytes was too small as hdr_v1 and hdr_v2 have increased the used header page areas to 1632 and 1648 bytes, respectively, so raise this to the minimum page size of 2048 and also make a variable for future ease
- do not return "not signed" for all caught exceptions, show StackTrace for future debugging then still return false for script purposes
- correct "test keys" boot image signing strings (scripts and app) to "verity keys"

app/src/main/java/com/topjohnwu/magisk/tasks/MagiskInstaller.kt

@@ -266,7 +266,7 @@ abstract class MagiskInstaller {
 
         val patched = File(installDir, "new-boot.img")
         if (isSigned) {
-            console.add("- Signing boot image with test keys")
+            console.add("- Signing boot image with verity keys")
             val signed = File(installDir, "signed.img")
             try {
                 withStreams(SuFileInputStream(patched), signed.outputStream().buffered()) {