Improve the error message when PyGMT fails to load the GMT library (#814)

When PyGMT fails to load the GMT library, it raises an error:
```
GMTCLibNotFoundError: Error loading the GMT shared library '/opt/GMT-6.1.1/lib/libgmt.dylib'.
```
However, it's still unclear why it fails.

After this PR, the error message will contain the original error
message. For example:
```
GMTCLibNotFoundError: Error loading the GMT shared library '/opt/GMT-6.1.1/lib/libgmt.dylib'.
 dlopen(/opt/GMT-6.1.1/lib/libgmt.dylib, 6): Library not loaded: @rpath/libnetcdf.15.dylib
  Referenced from: /opt/GMT-6.1.1/lib/libgmt.6.1.1.dylib
  Reason: image not found.
```

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

Author: seisman

pygmt/clib/loading.py

@@ -43,9 +43,8 @@ def load_libgmt():
             error = err
     if error:
         raise GMTCLibNotFoundError(
-            "Error loading the GMT shared library '{}':".format(
-                ", ".join(lib_fullnames)
-            )
+            "Error loading the GMT shared library "
+            f"{', '.join(lib_fullnames)}.\n {error}."
         )
     return libgmt