patch 8.2.1444: error messages are spread out and names can be confusing

Problem:    Error messages are spread out and names can be confusing.
Solution:   Start moving error messages to a separate file and use clear
            names.
diff --git a/Filelist b/Filelist
index 127e3e4..42eaa1d 100644
--- a/Filelist
+++ b/Filelist
@@ -45,6 +45,7 @@
 		src/drawline.c \
 		src/drawscreen.c \
 		src/edit.c \
+		src/errors.h \
 		src/eval.c \
 		src/evalbuffer.c \
 		src/evalfunc.c \