Explicitly identify the value that can't be converted to a date

[lognaturel]

In response to #405

What has been done to verify that this works as intended?

Added tests to make sure the exceptions are thrown when expected.

Why is this the best possible solution? Were any other approaches considered?

In Collect, these exception messages will be shown to the user in a dialog if re-computation leads to an invalid value being passed to a function that converts to date. These error messages should help the user identify what form design improvements they could make to avoid that case without making them too long. I considered providing suggestions like adding constraints or defaults but I think it's too hard to make generically useful.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

All this should do is improve error text.

Do we need any specific form for testing your changes? If so, please attach one.

decimal-date-time-bug.xls.zip

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

[codecov-io]

Codecov Report

Merging #494 into master will increase coverage by 0.01%.
The diff coverage is 57.14%.

@@             Coverage Diff              @@
##             master     #494      +/-   ##
============================================
+ Coverage     50.01%   50.02%   +0.01%     
- Complexity     3016     3017       +1     
============================================
  Files           247      247              
  Lines         13797    13797              
  Branches       2674     2674              
============================================
+ Hits           6900     6902       +2     
+ Misses         6051     6050       -1     
+ Partials        846      845       -1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/javarosa/xpath/expr/XPathFuncExpr.java	69.96% <57.14%> (+0.3%)	208 <0> (+1)	⬆️
...org/javarosa/core/model/condition/Triggerable.java	67.76% <0%> (-1.66%)	25% <0%> (ø)
.../java/org/javarosa/core/model/utils/DateUtils.java	58.33% <0%> (+0.57%)	72% <0%> (ø)	⬇️

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[ggalmazor]

LGTM.

I understand that this PR doesn't try to solve the core issue reported in #405.