fix confusing log in session.py

[MissCoconut]

androguard gives me a confusing log output when I tried to load a session.

{} already exists, overwriting!

Looking around session.py I found the weird log in line 46. , maybe it's better to print a more detailed log.

[codecov-io]

Codecov Report

Merging #828 (571ecb8) into master (99e48e7) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
- Coverage   76.83%   76.76%   -0.07%     
==========================================
  Files          51       51              
  Lines       16293    16284       -9     
==========================================
- Hits        12518    12501      -17     
- Misses       3775     3783       +8     

Impacted Files	Coverage Δ
androguard/session.py	55.85% <100.00%> (ø)
androguard/core/__init__.py	83.80% <0.00%> (-6.27%)	⬇️

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