UniversalPackages: Add better error handling when the web response doesn't return json

[dfederm]

A user is seeing the following error:

error MSB4018: The "DownloadUniversalPackages" task failed unexpectedly.
  System.Text.Json.JsonReaderException: '<' is an invalid start of a value. LineNumber: 2 | BytePositionInLine: 0.
     at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
     at System.Text.Json.Utf8JsonReader.ConsumeValue(Byte marker)
     at System.Text.Json.Utf8JsonReader.ReadFirstToken(Byte first)
     at System.Text.Json.Utf8JsonReader.ReadSingleSegment()
     at System.Text.Json.Utf8JsonReader.Read()
     at System.Text.Json.JsonDocument.Parse(ReadOnlySpan`1 utf8JsonSpan, JsonReaderOptions readerOptions, MetadataDb& database, StackRowStack& stack)
     at System.Text.Json.JsonDocument.Parse(ReadOnlyMemory`1 utf8Json, JsonReaderOptions readerOptions, Byte[] extraRentedArrayPoolBytes, PooledByteBufferWriter extraPooledByteBufferWriter)
     at System.Text.Json.JsonDocument.Parse(ReadOnlyMemory`1 json, JsonDocumentOptions options)
     at System.Text.Json.JsonDocument.Parse(String json, JsonDocumentOptions options)
     at Microsoft.Build.UniversalPackages.DownloadUniversalPackages.GetArtifactToolReleaseInfo(String osName, String arch, String patVar)
     at Microsoft.Build.UniversalPackages.DownloadUniversalPackages.GetArtifactToolPath(String patVar)
     at Microsoft.Build.UniversalPackages.DownloadUniversalPackages.Execute()
     at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
     at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext() [C:\__w\1\s\dirs.proj]

This change improves the error handling to provide a friendlier message and include the actual response to help debugging.

[dfederm]

/azp run

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