proper exception printing

[JonasIsensee]

For once a very simple and self-contained improvement:

#before
julia> throw(JLD2.InvalidDataException("test\ntest\n"))
ERROR: JLD2.InvalidDataException("test\ntest\n")
Stacktrace:
 [1] top-level scope
   @ REPL[33]:1

# after
julia> throw(JLD2.InvalidDataException("test\ntest\n"))
ERROR: InvalidDataException: test
test

Stacktrace:
 [1] top-level scope
   @ REPL[37]:1

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.34%. Comparing base (1de58ee) to head (3c5e4cb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/types.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
- Coverage   85.42%   85.34%   -0.08%     
==========================================
  Files          37       37              
  Lines        4439     4443       +4     
==========================================
  Hits         3792     3792              
- Misses        647      651       +4     

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