Make GetFailureMsgData format non-printable error messages

Some lightning implementations will send non-printable data in error
messages. For instance outdated versions of lnd will do this:

lightningnetwork/lnd@ff37b71

Whereas these are the error codes (sent as single-byte error messages):

https://github.com/lightningnetwork/lnd/blob/9b1ecbd3fa3fd93a6d95a5f71921c73777340760/lnwire/error.go#L12

These errors need to be printed as bytes when displayed to the user,
rather than interpretted as strings. Error.GetFailureMsgData will now
checks whether an error message is printable before returning the
message as a string and, if it's not, returns a string containing the
hex-encoded binary data in the error.

Author: canndrew

src/DotNetLightning.Core/Serialize/Msgs/Msgs.fs

@@ -1281,7 +1281,20 @@ type ErrorMessage =
                 ls.WriteWithLen(this.Data)
 
         member this.GetFailureMsgData() =
-            System.Text.ASCIIEncoding.ASCII.GetString(this.Data)
+            let minPrintableAsciiChar = 32uy
+            let isPrintableAsciiChar (asciiChar: byte) =
+                asciiChar >= minPrintableAsciiChar
+            let isPrintableAsciiString =
+                not <| Seq.exists
+                    (fun asciiChar -> not (isPrintableAsciiChar asciiChar))
+                    this.Data
+            if isPrintableAsciiString then
+                System.Text.ASCIIEncoding.ASCII.GetString this.Data
+            else
+                Seq.fold
+                    (fun msg (asciiChar: byte) -> sprintf "%s %02x" msg asciiChar)
+                    "<error contains non-printable binary data>:"
+                    this.Data
 
 and WhichChannel =
     | SpecificChannel of ChannelId