Catch subprocess.CalledProcessError if 'gmt --show-library' fails (#974)

The `gmt --show-library` command may fail due to linking with a broken
library. For example, if my netcdf library is missing, running `gmt
--show-library` will raise:
```
dyld: Library not loaded: /usr/local/opt/netcdf/lib/libnetcdf.18.dylib
  Referenced from: /Users/seisman/opt/GMT-master/bin/gmt
  Reason: image not found
[1]    84067 abort      gmt
```
`subprocess.check_output` will raise the `CalledProcessError` exception.

This PR catches the exception.

Author: seisman

pygmt/clib/loading.py

@@ -116,7 +116,10 @@ def clib_full_names(env=None):
         )
         assert libfullpath.exists()
         yield str(libfullpath)
-    except (FileNotFoundError, AssertionError):  # command not found
+    except (FileNotFoundError, AssertionError, sp.CalledProcessError):
+        # the 'gmt' executable  is not found
+        # the gmt library is not found
+        # the 'gmt' executable is broken
         pass
 
     # 3. Search for DLLs in PATH by calling find_library() (Windows only)