[generator] Avoid 'error (…):' construct in diagnostic messages (#851)

Context: #850

When a parsing issue is hit when importing Javadoc info, an "error"
is printed out like this:

	Error (31:41): Syntax error, expected: #PCDATA, <tt>, <TT>, <i>, <I>, {@code, {@docroot}, {@inheritdoc}, {@link, {@linkplain, {@literal, {@value}, {@value, UnknownHtmlElementStart, </tt>, </TT>, </i>, </I>, </p>, </P>, <p>, <P>, <pre>, <PRE>, @author, @apiSince, @deprecated, @deprecatedSince, @exception, @param, @return, @see, @Serialdata, @serialField, @SInCE, @throws, @[unknown], @Version
	    {@link #getCurrentTrackSelections()}}.</li>

This is intended to be informational, but this output format triggers
the MSBuild error parsing regex, and is interpreted as an actual
error, causing the build to fail.

Avoid the error by prepending `JavadocImport-` to the
`LogMessage.Level` enum value, so that MSBuild doesn't interpret the
string as an error and the build can successfully complete:

	JavadocImport-Error (31:41): Syntax error, expected: #PCDATA, <tt>, <TT>, <i>, <I>, {@code, {@docroot}, {@inheritdoc}, {@link, {@linkplain, {@literal, {@value}, {@value, UnknownHtmlElementStart, </tt>, </TT>, </i>, </I>, </p>, </P>, <p>, <P>, <pre>, <PRE>, @author, @apiSince, @deprecated, @deprecatedSince, @exception, @param, @return, @see, @Serialdata, @serialField, @SInCE, @throws, @[unknown], @Version
	    {@link #getCurrentTrackSelections()}}.</li>

tools/generator/Java.Interop.Tools.Generator.ObjectModel/JavadocInfo.cs

@@ -186,7 +186,7 @@ static void PrintMessages (ParseTree tree, TextWriter writer)
  {
  var lines = GetLines (tree.SourceText);
  foreach (var m in tree.ParserMessages) {
- writer.WriteLine ($"{m.Level} {m.Location}: {m.Message}");
+ writer.WriteLine ($"JavadocImport-{m.Level} {m.Location}: {m.Message}");
  writer.WriteLine (lines [m.Location.Line]);
  writer.Write (new string (' ', m.Location.Column));
  writer.WriteLine ("^");