Catch subprocess.CalledProcessError if 'gmt --show-library' fails (#974)
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.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python