Exit with error if target is missing

Up until now it was possible to request a target that didn't exist
in the core description file, and FuseSoC would treat it basically
as an empty target and most likely fail later with a cryptic error
message. This was originally done as a compromise to provide
backwards-compatibility with the CAPI1 format, but since we no longer
support that, let's provide the user with a more sensible error
message instead.

fusesoc/capi2/core.py

@@ -172,6 +172,8 @@ def get_flags(self, target_name):
                     # Special case for tool as we get it from default_tool
                     flags["tool"] = str(target["default_tool"])
 
+        else:
+            raise RuntimeError(f"'{self.name}' has no target '{target_name}'")
         return flags
 
     def get_flow(self, flags):

fusesoc/main.py

@@ -304,6 +304,9 @@ def run(fs, args):
     except SyntaxError as e:
         logger.error(str(e))
         exit(1)
+    except RuntimeError as e:
+        logger.error(str(e))
+        exit(1)
 
     # Unconditionally clean out the work root on fresh builds
     # if we use the old tool API

tests/test_capi2.py

@@ -280,7 +280,9 @@ def test_capi2_get_flags():
 
     core = Core(Core2Parser(), os.path.join(cores_dir, "flags.core"))
 
-    assert {} == core.get_flags("bad_target")
+    with pytest.raises(RuntimeError) as e:
+        core.get_flags("bad_target")
+    assert "::flags:0' has no target 'bad_target'" in str(e.value)
     assert {} == core.get_flags("noflags")
     assert {} == core.get_flags("emptyflags")
     assert {"tool": "mytool"} == core.get_flags("emptyflagstool")