Should fail gracefully without write access to disk

[john-g-g]

Not all library operations require keys and/or a wallet, eg, streaming blocks from blockchain.

hive_1   | Traceback (most recent call last):                                      
hive_1   |   File "/app/hive/server/serve.py", line 13, in <module>                
hive_1   |     from hive.indexer.core import db_last_block, head_state             
hive_1   |   File "/app/hive/indexer/core.py", line 11, in <module>                
hive_1   |     from steem.blockchain import Blockchain                             
hive_1   |   File "/usr/local/lib/python3.5/dist-packages/steem/__init__.py",   
line 2, in <module>                                                                
hive_1   |     from .steem import Steem                                            
hive_1   |   File "/usr/local/lib/python3.5/dist-packages/steem/steem.py", line 
1, in <module>                                                                     
hive_1   |     from .commit import Commit                                          
hive_1   |   File "/usr/local/lib/python3.5/dist-packages/steem/commit.py",        
line 18, in <module>                                                               
hive_1   |     from steembase.storage import configStorage                         
hive_1   |   File                                                                  
"/usr/local/lib/python3.5/dist-packages/steembase/storage.py", line 459, in        
<module>                                                                           
hive_1   |     keyStorage = Key()                                                  
hive_1   |   File                                                                  
"/usr/local/lib/python3.5/dist-packages/steembase/storage.py", line 114, in        
__init__                                                                           
hive_1   |     super(Key, self).__init__()                                         
hive_1   |   File                                                                  
"/usr/local/lib/python3.5/dist-packages/steembase/storage.py", line 51, in         
__init__                                                                           
hive_1   |     self.mkdir_p()                                                      
hive_1   |   File                                                                  
"/usr/local/lib/python3.5/dist-packages/steembase/storage.py", line 61, in         
mkdir_p                                                                            
hive_1   |     os.makedirs(self.data_dir)                                          
hive_1   |   File "/usr/lib/python3.5/os.py", line 231, in makedirs                
hive_1   |     makedirs(head, mode, exist_ok)                                      
hive_1   |   File "/usr/lib/python3.5/os.py", line 231, in makedirs                
hive_1   |     makedirs(head, mode, exist_ok)                                      
hive_1   |   File "/usr/lib/python3.5/os.py", line 231, in makedirs                
hive_1   |     makedirs(head, mode, exist_ok)                                      
hive_1   |   File "/usr/lib/python3.5/os.py", line 241, in makedirs                
hive_1   |     mkdir(name, mode)                                                   
hive_1   | PermissionError: [Errno 13] Permission denied: '/nonexistent'