Detect when gobject-introspection is missing

Instead of a compiler bug error, show a proper error message that asks
whether gobject-introspection is installed.

Fixes #58.

blueprintcompiler/ast_utils.py

@@ -81,6 +81,8 @@ class AstNode:
                 validator(self)
             except CompileError as e:
                 yield e
+                if e.fatal:
+                    return
 
         for child in self.children:
             yield from child._get_errors()

blueprintcompiler/errors.py

@@ -37,7 +37,7 @@ class CompileError(PrintableError):
     category = "error"
     color = Colors.RED
 
-    def __init__(self, message, start=None, end=None, did_you_mean=None, hints=None, actions=None):
+    def __init__(self, message, start=None, end=None, did_you_mean=None, hints=None, actions=None, fatal=False):
         super().__init__(message)
 
         self.message = message
@@ -45,6 +45,7 @@ class CompileError(PrintableError):
         self.end = end
         self.hints = hints or []
         self.actions = actions or []
+        self.fatal = fatal
 
         if did_you_mean is not None:
             self._did_you_mean(*did_you_mean)

blueprintcompiler/language/imports.py

@@ -39,6 +39,11 @@ class GtkDirective(AstNode):
                 err.hint("Expected 'using Gtk 4.0;'")
             raise err
 
+        try:
+            gir.get_namespace("Gtk", self.tokens["version"])
+        except:
+            raise CompileError("Could not find GTK 4 introspection files. Is gobject-introspection installed?", fatal=True)
+
 
     @property
     def gir_namespace(self):