Fix: Return entire stack trace for deadline exceeded error

[zoercai]

Cloud Spanner JDBC Driver stack trace is not clear on cause of the issue when there is a DEADLINE_EXCEEDED error, making it more difficult for the customer and Cloud Support to debug the issue. This PR modifies the exception mapping to return the original exception rather than the message only, so that the entire stack trace is shown.

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[codecov]

Codecov Report

Merging #347 (b82b420) into master (4daa18d) will increase coverage by 0.62%.
The diff coverage is 78.43%.

@@             Coverage Diff              @@
##             master     #347      +/-   ##
============================================
+ Coverage     71.35%   71.98%   +0.62%     
- Complexity     1114     1131      +17     
============================================
  Files            24       24              
  Lines          3474     3520      +46     
  Branches        531      537       +6     
============================================
+ Hits           2479     2534      +55     
+ Misses          778      767      -11     
- Partials        217      219       +2     

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/cloud/spanner/jdbc/JdbcDriver.java	62.85% <ø> (ø)	15.00 <0.00> (ø)
...le/cloud/spanner/jdbc/JdbcSqlExceptionFactory.java	44.44% <0.00%> (-1.27%)	7.00 <0.00> (ø)
.../java/com/google/cloud/spanner/jdbc/JdbcArray.java	71.81% <83.33%> (+15.75%)	28.00 <14.00> (+15.00)
...a/com/google/cloud/spanner/jdbc/JdbcResultSet.java	75.34% <0.00%> (+0.54%)	190.00% <0.00%> (+2.00%)
...va/com/google/cloud/spanner/jdbc/JdbcDataType.java	75.80% <0.00%> (+17.74%)	4.00% <0.00%> (ø%)

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