Vim plugin: prefix messages with "Black: " (psf#3194)

As mentioned in psfGH-3185, when using Black as a Vim plugin, especially
automatically on save, the plugin's messages can be confusing, as
nothing indicates that they come from Black.

CHANGES.md

@@ -46,6 +46,8 @@
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
 
+- Vim plugin: prefix messages with `Black: ` so it's clear they come from Black (#3194)
+
 ### Output
 
 <!-- Changes to Black's terminal output and error messages -->

autoload/black.vim

@@ -158,9 +158,9 @@ def Black(**kwargs):
     )
   except black.NothingChanged:
     if not quiet:
-      print(f'Already well formatted, good job. (took {time.time() - start:.4f}s)')
+      print(f'Black: already well formatted, good job. (took {time.time() - start:.4f}s)')
   except Exception as exc:
-    print(exc)
+    print(f'Black: {exc}')
   else:
     current_buffer = vim.current.window.buffer
     cursors = []
@@ -177,7 +177,7 @@ def Black(**kwargs):
       except vim.error:
         window.cursor = (len(window.buffer), 0)
     if not quiet:
-      print(f'Reformatted in {time.time() - start:.4f}s.')
+      print(f'Black: reformatted in {time.time() - start:.4f}s.')
 
 def get_configs():
   filename = vim.eval("@%")