Correct spelling in error messages and translations

Correct spelling of occured (sic) in three error messages:
 1. An I/O error occured while sending to the backend.
 1. Something unusual has occurred to cause the driver to fail
 1. An error occurred while setting up the SSL connection

Rationale:
  - When using the pgjdbc adapter my project would like to display the adapter's error message to the user and the misspelling is embarrassing.
  -  Since the English messages are embedded in the Java code, we need to correct the strings in the code. I also updated the translation keys in the *.po files.
 
(Revision of https://github.com/pgjdbc/pgjdbc/pull/34)