Use failOnError instead of try! in Swift

Replace existing usages of try! with the failOnError helper. Add guidance to 
the coding guidelines doc.

CODING_GUIDELINES.md

@@ -73,6 +73,8 @@ These should usually be prioritized in that order, but adjust the trade-off as n
 
 - **Write API docs** using [DocC syntax][] (a Markdown dialect), unless an API is trivial (or not app-team-facing). Even for internal methods, though, if you do write a comment, make it a doc comment (like for Rust code), because it shows up in IDEs.
 
+- To make sure that error messages get into logs, we use the `failOnError` helper instead of `try!` for forcing an unwrap on the result of an operation that can throw an error.
+
 [DocC syntax]: https://www.swift.org/documentation/docc/writing-symbol-documentation-in-your-source-files
 
 

swift/Sources/LibSignalClient/MessageBackup.swift

@@ -75,9 +75,11 @@ public struct MessageBackupUnknownFields {
 
 private class ValidationOutcome: NativeHandleOwner {
   public var unknownFields: MessageBackupUnknownFields {
-    let fields = try! self.withNativeHandle { result in
-      try invokeFnReturningStringArray {
-        signal_message_backup_validation_outcome_get_unknown_fields($0, result)
+    let fields = failOnError {
+      try self.withNativeHandle { result in
+        try invokeFnReturningStringArray {
+          signal_message_backup_validation_outcome_get_unknown_fields($0, result)
+        }
       }
     }
     return MessageBackupUnknownFields(fields: fields)

swift/Sources/LibSignalClient/SealedSenderCertificates.swift

@@ -198,7 +198,9 @@ public class SenderCertificate: NativeHandleOwner {
     }
 
     public var sender: SealedSenderAddress {
-        return try! SealedSenderAddress(e164: self.senderE164, uuidString: self.senderUuid, deviceId: self.deviceId)
+        return failOnError {
+            try SealedSenderAddress(e164: self.senderE164, uuidString: self.senderUuid, deviceId: self.deviceId)
+        }
     }
 
     public var serverCertificate: ServerCertificate {