fail to generate docs if the code isn't there

tools/copy_from_GSW-C.py

@@ -20,6 +20,12 @@
 srcdir = Path('..', '..', 'GSW-C')
 destdir = Path('..', 'src', 'c_gsw')
 
+if not srcdir.exists():
+    raise IOError(
+        f"Could not find the GSW-C source code in {srcdir}."
+        "Please read the development notes to find how to setup your GSW-Python development environment."
+        )
+
 for fname in fnames:
     src = srcdir.joinpath(fname)
     dest = destdir.joinpath(fname)

tools/matlab_parser.py

@@ -9,6 +9,12 @@
 basedir = Path('..').resolve()
 
 gsw_matlab_dir = basedir.joinpath('..', 'GSW-Matlab', 'Toolbox').resolve()
+if not gsw_matlab_dir.exists():
+    raise IOError(
+        f"Could not find the GSW-Matlab source code in {gsw_matlab_dir}."
+        "Please read the development notes to find how to setup your GSW-Python development environment."
+        )
+
 gsw_matlab_subdirs = ['library', 'thermodynamics_from_t']
 
 # pattern for functions returning one variable