Fix importer bug and error reporting problem

Two issues:
1. File imports were not using the stream wrapper, so imported files needed spaces between `!else` and the following `:`.
2. An exception during the actual Ruamel parsing of a file import would end up resulting in a later exception insinuating that a circular import had occurred. A circular import should never actually occur... as far as I know it can't. Something would need to force the system to evaluate parts of the Yamlet logic at load time, which is expressly not how Yamlet works.

yamlet.py

@@ -170,7 +170,11 @@ def LoadCachedFile(self, fn):
                              'are deferred until name lookup.')
       return res
     self.loaded_modules[fn] = None
-    with open(fn) as file: res = self._ProcessYamlGcl(file)
+    with open(fn) as file:
+      try: res = self._ProcessYamlGcl(file)
+      except Exception as e:
+        self.loaded_modules.pop(fn)
+        raise
     self.loaded_modules[fn] = res
     return res
 
@@ -208,7 +212,7 @@ def _ConstructElse(loader, node):
         f'Yamlet `!else` got unexpected node type: {repr(node)}')
 
   def _ProcessYamlGcl(self, ygcl):
-    tup = super().load(ygcl)
+    tup = super().load(_WrapStream(ygcl))
     ectx = None
     while isinstance(tup, DeferredValue):
       if not ectx:
@@ -219,7 +223,7 @@ def _ProcessYamlGcl(self, ygcl):
 
   def load_file(self, filename):
     with open(filename) as fn: return self.load(fn)
-  def load(self, yaml_gcl): return self._ProcessYamlGcl(_WrapStream(yaml_gcl))
+  def load(self, yaml_gcl): return self._ProcessYamlGcl(yaml_gcl)
 
 
 class _DebugOpts:
@@ -652,9 +656,9 @@ def _gcl_evaluate_(self, value, ectx):
     if not fn.exists():
       if value == fn:
         ectx.Raise(FileNotFoundError,
-                   f'Could not import YamlBcl file: {value}')
+                   f'Could not import Yamlet file: {value}')
       ectx.Raise(FileNotFoundError,
-                 f'Could not import YamlBcl file: `{fn}`\n'
+                 f'Could not import Yamlet file: `{fn}`\n'
                  f'As evaluated from this expression: `{value}`.\n')
     loaded = self._gcl_loader_(fn)
     return loaded