[python]: Improve build error message when building non-fabricated pa…

…ckage

Instead of outputting a ModuleNotFoundError when attempting to build the
cocoex Python package without running `scripts/fabricate` beforehand,
output an informative error message that tells you how to resolve the
issue.

Fixes #1

build/python/setup.py

@@ -1,6 +1,36 @@
+import sys
 import numpy as np
+
+from pathlib import Path
 from setuptools import Extension, setup
 
+
+## Ensure source files are present:
+BASE_DIR = Path(__file__).parent.absolute()
+COCOEX_DIR = Path("src") / "cocoex"
+MISSING = []
+for fname in ["coco.c", "coco.h"]:
+    full_fname = BASE_DIR / COCOEX_DIR / fname
+    if not full_fname.exists():
+        MISSING.append(str(COCOEX_DIR / fname))
+
+if len(MISSING) > 0:
+    print(f"""BUILD ERROR
+
+Missing source files: {", ".join(MISSING)}
+    
+Could not find some generated source files. These are required to build the 
+Python package. If you are building in-tree from a git checkout of the 
+'numbbo/coco-experiment' repository, did you forget to run the 
+`scripts/fabricate` script to generate the COCO source bundles?
+
+If you are not building in-tree or ran `scripts/fabricate`, please file an 
+issue on Github: https://github.com/numbbo/coco-experiment/issues
+""")
+    sys.exit(-1)
+
+
+## Configure Cython build
 extensions = []
 extensions.append(Extension(name="cocoex.interface",
                             sources=["src/cocoex/coco.c", "src/cocoex/interface.pyx"],