pw_protobuf_compiler: GN fixes

- Use += instead of = for GN list.
- Fix capitalization of pw_protobuf_GENERATORS variable in warning.

Change-Id: I8d2028fe37038e778b955f768c4db188289c181e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/14342
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 7eba431..935f081 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -365,7 +365,7 @@
     }
 
     if (defined(invoker.deps)) {
-      _gen_deps = process_file_template(invoker.deps, "{{source}}_${_gen}")
+      _gen_deps += process_file_template(invoker.deps, "{{source}}_${_gen}")
 
       if (_gen == "nanopb_rpc") {
         # RPC dependencies also depend on their core generated protos.
diff --git a/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py b/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py
index 33e24b7..e089cdb 100644
--- a/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py
+++ b/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py
@@ -32,7 +32,7 @@
     print(f'ERROR: Attempting to build protobuf library {library}, but the '
           f'{generator} protobuf generator is not in use.')
     print(f'To use {generator} protobufs, list "{generator}" in '
-          'pw_protobuf_generators.')
+          'pw_protobuf_GENERATORS.')
     sys.exit(1)