LD_LIBRARY_PATH-induced Python import errors: Shorten error message

[tswilliams]

If the uHAL Python module cannot be imported due to LD_LIBRARY_PATH missing relevant paths (e.g. /opt/cactus/lib for a standard install), then right now the following message will be printed

ImportError: libcactus_uhal_tests.so.2.7: cannot open shared object file: No such file or directory
N.B. ImportError (or derived exception) raised when uHAL's __init__.py tries to load python bindings library
     Maybe you need to add "/opt/cactus/lib", or some other path, to the "LD_LIBRARY_PATH" environment variable?

On reflection, this is a bit too verbose and should be shortened to something like:

ImportError: Could not load uHAL Python bindings (typical cause: paths missing from LD_LIBRARY_PATH). Details: "libcactus_uhal_tests.so.2.7: cannot open shared object file: No such file or directory"

Also: We should update gen_ipbus_addr_decode to make sure its output in this scenario is more concise & user-friendly. Current output:

--(serenity_ku15p_so1)--> /opt/cactus/bin/uhal/tools/gen_ipbus_addr_decode aaa
Traceback (most recent call last):
  File "/opt/cactus/bin/uhal/tools/gen_ipbus_addr_decode", line 22, in <module>
    import uhal
  File "/usr/lib/python2.7/site-packages/uhal/__init__.py", line 14, in <module>
    exec('raise type(e), type(e)(e.message + msg_suffix), sys.exc_info()[2]')
  File "/usr/lib/python2.7/site-packages/uhal/__init__.py", line 5, in <module>
    from ._core import *
ImportError: libcactus_uhal_tests.so.2.7: cannot open shared object file: No such file or directory
N.B. ImportError (or derived exception) raised when uHAL's __init__.py tries to load python bindings library
     Maybe you need to add "/opt/cactus/lib", or some other path, to the "LD_LIBRARY_PATH" environment variable?