Pipeline instanciation: better error message when an inverse operatio…

…n is needed in forward path and not available

and fix a copy-paste error where fwd4d was tested instead of inv4d

Before:
```
$ echo 0 0 | bin/cs2cs +proj=isea +to +proj=longlat
pipeline: Pipeline: A forward operation couldn't be constructed
```

Now:
```
$ echo 0 0 | bin/cs2cs +proj=isea +to +proj=longlat
pipeline: Pipeline: Inverse operation for isea is not available
```

Slightly related to OSGeo/gdal#9149

src/pipeline.cpp

@@ -533,12 +533,24 @@ PJ *OPERATION(pipeline, 0) {
     /* Require a forward path through the pipeline */
     for (auto &step : pipeline->steps) {
         PJ *Q = step.pj;
-        if ((Q->inverted && (Q->inv || Q->inv3d || Q->fwd4d)) ||
-            (!Q->inverted && (Q->fwd || Q->fwd3d || Q->fwd4d))) {
+        if (step.omit_fwd) {
             continue;
+        }
+        if (Q->inverted) {
+            if (Q->inv || Q->inv3d || Q->inv4d) {
+                continue;
+            }
+            proj_log_error(
+                P, _("Pipeline: Inverse operation for %s is not available"),
+                Q->short_name);
+            return destructor(P, PROJ_ERR_OTHER_NO_INVERSE_OP);
         } else {
+            if (Q->fwd || Q->fwd3d || Q->fwd4d) {
+                continue;
+            }
             proj_log_error(
-                P, _("Pipeline: A forward operation couldn't be constructed"));
+                P, _("Pipeline: Forward operation for %s is not available"),
+                Q->short_name);
             return destructor(P, PROJ_ERR_INVALID_OP_WRONG_SYNTAX);
         }
     }

test/gie/more_builtins.gie

@@ -920,10 +920,10 @@ expect      5.875      55.375      0
 # |                |       invertible step       |     non-invertible step     |
 # | flags          | forward path | inverse path | forward path | inverse path |
 # | -------------- | ------------ | ------------ | ------------ | ------------ |
-# | +omit_fwd      | omit         | inv          | omit         | undefined    |
+# | +omit_fwd      | omit         | inv          | omit         | runtime err  |
 # | +omit_fwd +inv | omit         | fwd          | omit         | fwd          |
 # | +omit_inv      | fwd          | omit         | fwd          | omit         |
-# | +omit_inv +inv | inv          | omit         | undefined    | omit         |
+# | +omit_inv +inv | inv          | omit         | pipeline creation error     |
 #
 # From the table we can see that invertible steps should work pretty much all
 # the time. Non-invertible steps on the other hand make either the forward path
@@ -1033,20 +1033,13 @@ direction inverse
 accept 1 2 3
 expect 1 2 3
 
-# Test that the forward path is not defined and inverse path does nothing.
+# Test that the forward path is not defined
 #
 # When going through the forward path, inv specifies that we should execute the
 # step in reverse. Because the affine transformation does not have an inverse,
 # this means that the forward path does not exist.
 
 operation proj=pipeline step proj=affine s11=1 s12=1 s13=1 s22=0 s33=0 omit_inv inv
-
-direction forward
-accept 1 2 3
 expect failure errno no_inverse_op
 
-direction inverse
-accept 1 2 3
-expect 1 2 3
-
 </gie-strict>