Added file and line to NIOAsyncWriterError description (#2693)

Motivation:

Even though NIOAsyncWriterError captures the file and line of the error they are internal and not printed in the
description.

Modifications:

- made the 2 properties public
- added them in the description

Result:

Users will be able to access and see the line number and file where a NIOAsyncWriterError occurred

Sources/NIOCore/AsyncSequences/NIOAsyncWriter.swift

@@ -80,11 +80,9 @@ public struct NIOAsyncWriterError: Error, Hashable, CustomStringConvertible {
  @usableFromInline
  let _code: _Code
 
- @usableFromInline
- var file: String
+ public var file: String
 
- @usableFromInline
- var line: Int
+ public var line: Int
 
  @inlinable
  init(_code: _Code, file: String, line: Int) {
@@ -111,7 +109,7 @@ public struct NIOAsyncWriterError: Error, Hashable, CustomStringConvertible {
 
  @inlinable
  public var description: String {
- "NIOAsyncWriterError.\(self._code.rawValue)"
+ "NIOAsyncWriterError.\(self._code.rawValue): \(self.file):\(self.line)"
  }
 }