Fix error message in C# xmlization on declarations

When we encountered XML declarations, we threw exceptions with
uninformative messages. Namely, we expect the reader to be moved to the
content, but most users omitted to read that in the documentation.

We refine the error message for this particular situation, and hint at
`MoveToContent` method on the reader.

See [this aas-core3.0-csharp issue] for more details.

[this aas-core3.0-csharp issue]: aas-core-works/aas-core3.0-csharp#24

aas_core_codegen/csharp/xmlization/_generate.py

@@ -1205,6 +1205,17 @@ def _generate_deserialize_from(name: Identifier) -> Stripped:
 public static Aas.{name} {name}From(
 {I}Xml.XmlReader reader)
 {{
+{I}DeserializeImplementation.SkipNoneWhitespaceAndComments(reader);
+
+{I}if (!reader.EOF && reader.NodeType == Xml.XmlNodeType.XmlDeclaration)
+{I}{{
+{II}throw new Xmlization.Exception(
+{III}"",
+{III}"Unexpected XML declaration when reading an instance " +
+{III}"of class {name}, as we expect the reader " +
+{III}"to be set at content with MoveToContent");
+{I}}}
+
 {I}Aas.{name}? result = (
 {II}DeserializeImplementation.{name}FromElement(
 {III}reader,
@@ -1879,7 +1890,7 @@ def generate(
 {I}public readonly string Path;
 {I}public readonly string Cause;
 {I}public Exception(string path, string cause)
-{II}: base($"{{cause}} at: {{path}}")
+{II}: base($"{{cause}} at: {{(path == "" ? "the beginning" : path)}}")
 {I}{{
 {II}Path = path;
 {II}Cause = cause;