Add more context to quit_if_file_exists in configure.py

Currently, having a dirty `obj/` directory is sufficient to abort CI
tests. This results in errors like the following:

```
...
== end clock drift check ==
sccache: Starting the server...
configure: error: Existing 'config.toml' detected.
== clock drift check ==
...
```

This is subtle and doesn't give a good idea as to what causes the issue.
With this patch, the error becomes more prominent and a resolution is
suggested:

```
== end clock drift check ==
sccache: Starting the server...

configure: ERROR: Existing 'config.toml' detected. Exiting
Is objdir '/home/tmgross/projects/rust/obj' clean?

== clock drift check ==
```

src/bootstrap/configure.py

@@ -180,7 +180,7 @@ def p(msg):
 
 
 def err(msg):
-    print("configure: error: " + msg)
+    print("\nconfigure: ERROR: " + msg + "\n")
     sys.exit(1)
 
 def is_value_list(key):
@@ -544,7 +544,14 @@ def write_config_toml(writer, section_order, targets, sections):
 
 def quit_if_file_exists(file):
     if os.path.isfile(file):
-        err("Existing '" + file + "' detected.")
+        msg = "Existing '{}' detected. Exiting".format(file)
+
+        # If the output object directory isn't empty, we can get these errors
+        host_objdir = os.environ.get("OBJDIR_ON_HOST")
+        if host_objdir is not None:
+            msg += "\nIs objdir '{}' clean?".format(host_objdir)
+
+        err(msg)
 
 if __name__ == "__main__":
     # If 'config.toml' already exists, exit the script at this point