patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/normal.c b/src/normal.c
index b2d0c01..18f14ec 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -5210,7 +5210,7 @@
     else if (!checkclearopq(cap->oap))
     {
 	if (!curbuf->b_p_ma)
-	    emsg(_(e_modifiable));
+	    emsg(_(e_cannot_make_changes_modifiable_is_off));
 	else
 	{
 	    if (virtual_active())
@@ -5235,7 +5235,7 @@
     else if (!checkclearopq(cap->oap))
     {
 	if (!curbuf->b_p_ma)
-	    emsg(_(e_modifiable));
+	    emsg(_(e_cannot_make_changes_modifiable_is_off));
 	else
 	{
 	    if (cap->extra_char == Ctrl_V)	// get another character
@@ -7075,7 +7075,7 @@
     else if (!curbuf->b_p_ma && !p_im)
     {
 	// Only give this error when 'insertmode' is off.
-	emsg(_(e_modifiable));
+	emsg(_(e_cannot_make_changes_modifiable_is_off));
 	clearop(cap->oap);
 	if (cap->cmdchar == K_PS)
 	    // drop the pasted text