Improve the error message for loading a old version of the GMT library (#925)

Co-authored-by: Wei Ji <[email protected]>

Author: seisman

pygmt/clib/loading.py

@@ -151,5 +151,11 @@ def check_libgmt(libgmt):
     functions = ["Create_Session", "Get_Enum", "Call_Module", "Destroy_Session"]
     for func in functions:
         if not hasattr(libgmt, "GMT_" + func):
-            msg = f"Error loading libgmt. Couldn't access function GMT_{func}."
+            # pylint: disable=protected-access
+            msg = (
+                f"Error loading '{libgmt._name}'. Couldn't access function GMT_{func}. "
+                "Ensure that you have installed an up-to-date GMT version 6 library. "
+                "Please set the environment variable 'GMT_LIBRARY_PATH' to the "
+                "directory of the GMT 6 library."
+            )
             raise GMTCLibError(msg)

pygmt/tests/test_clib_loading.py

@@ -13,8 +13,21 @@ def test_check_libgmt():
     """
     Make sure check_libgmt fails when given a bogus library.
     """
-    with pytest.raises(GMTCLibError):
-        check_libgmt(dict())
+    # create a fake library with a "_name" property
+    def libgmt():
+        pass
+
+    libgmt._name = "/path/to/libgmt.so"  # pylint: disable=protected-access
+    msg = (
+        # pylint: disable=protected-access
+        f"Error loading '{libgmt._name}'. "
+        "Couldn't access function GMT_Create_Session. "
+        "Ensure that you have installed an up-to-date GMT version 6 library. "
+        "Please set the environment variable 'GMT_LIBRARY_PATH' to the "
+        "directory of the GMT 6 library."
+    )
+    with pytest.raises(GMTCLibError, match=msg):
+        check_libgmt(libgmt)
 
 
 def test_load_libgmt():