fix(excmd): nlua_call_excmd require() failure is a "lua_error"

Although `nlua_call_excmd` is semantically for implementing Ex-commands,
the `require()` should never fail, so that's a "Lua error".

But if the call itself fails (the later `semsg` call), that's an "Ex
cmd" error.

scripts/linterrcodes.lua

@@ -48,7 +48,7 @@ local dup_allowed = {
   E509 = 2,
   E5101 = 2,
   E5102 = 2,
-  E5108 = 6,
+  E5108 = 5,
   E5111 = 2,
   E513 = 2,
   E521 = 2,

src/nvim/ex_eval.c

@@ -378,7 +378,9 @@ bool do_intthrow(cstack_T *cstack)
   return true;
 }
 
-/// Get an exception message that is to be stored in current_exception->value.
+/// Gets an exception message that is to be stored in current_exception->value.
+///
+/// For error exceptions (ET_ERROR), formats the message as "Vim(cmdname):Exx: …".
 char *get_exception_string(void *value, except_type_T type, char *cmdname, bool *should_free)
 {
   char *ret;

src/nvim/lua/executor.c

@@ -1785,8 +1785,7 @@ bool nlua_call_excmd(const char *module, const char *func, exarg_T *eap, const c
   lua_getglobal(lstate, "require");
   lua_pushstring(lstate, module);
   if (lua_pcall(lstate, 1, 1, 0) != 0) {
-    semsg("E5108: %s", lua_tostring(lstate, -1));
-    lua_pop(lstate, 1);
+    nlua_error(lstate, "E5108: %s");
     return false;
   }
   lua_getfield(lstate, -1, func);
@@ -1802,7 +1801,8 @@ bool nlua_call_excmd(const char *module, const char *func, exarg_T *eap, const c
   }
 
   if (nlua_pcall(lstate, nargs, 0)) {
-    semsg("E5108: %s", lua_tostring(lstate, -1));
+    // Not "E5108" because this is a logical/application error, not a "Lua error".
+    emsg(lua_tostring(lstate, -1));
     lua_pop(lstate, 1);
     return false;
   }

test/functional/ex_cmds/excmd_spec.lua

@@ -8,7 +8,20 @@ local fn = n.fn
 local pcall_err = t.pcall_err
 local assert_alive = n.assert_alive
 
-describe('Ex cmds', function()
+describe('nlua_call_excmd excmds', function()
+  -- Exercise nlua_call_excmd by testing commands implemented with it (:log, :lsp).
+
+  before_each(function()
+    clear()
+  end)
+
+  it('error propagation, formatting', function()
+    t.eq('Vim(lsp):E5800: Invalid :lsp subcommand: bogus', pcall_err(command, 'lsp bogus'))
+    t.matches('Vim%(log%):E5200: No such log.*', pcall_err(command, 'log bogus'))
+  end)
+end)
+
+describe('excmds', function()
   before_each(function()
     clear()
   end)

test/functional/ex_cmds/lsp_spec.lua

@@ -29,7 +29,7 @@ describe(':lsp', function()
       env = { VIMRUNTIME = 'non-existent' },
     }
     t.matches(
-      [[Vim%(lsp%):E%d+: .*module 'vim%.lsp' not found:]],
+      [[.*module 'vim%.lsp' not found:]],
       vim.split(t.pcall_err(n.command, 'lsp enable dummy'), '\n')[1]
     )
   end)