HighlightingAssets: Make .syntaxes() and syntax_for_file_name() failable

Or rather, introduce new versions of these methods and deprecate the old ones.

This is preparation to enable robust and user-friendly support for lazy-loading.
With lazy-loading, we don't know if the SyntaxSet is valid until after we try to
use it, so wherever we try to use it, we need to return a Result. See discussion
about panics in #1747.

src/printer.rs

@@ -174,7 +174,7 @@ impl<'a> InteractivePrinter<'a> {
             let syntax = match assets.get_syntax(config.language, input, &config.syntax_mapping) {
                 Ok(syntax) => syntax,
                 Err(Error(ErrorKind::UndetectedSyntax(_), _)) => {
-                    assets.get_syntax_set().find_syntax_plain_text()
+                    assets.get_syntax_set()?.find_syntax_plain_text()
                 }
                 Err(e) => return Err(e),
             };
@@ -192,7 +192,7 @@ impl<'a> InteractivePrinter<'a> {
             #[cfg(feature = "git")]
             line_changes,
             highlighter,
-            syntax_set: assets.get_syntax_set(),
+            syntax_set: assets.get_syntax_set()?,
             background_color_highlight,
         })
     }