Update UG , DG and improve error message

[Diwu-Yi]

No description provided.

[codecov-io]

Codecov Report

Merging #254 (35861c5) into master (9f5f3de) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #254   +/-   ##
=========================================
  Coverage     66.65%   66.65%           
  Complexity      738      738           
=========================================
  Files           124      124           
  Lines          2498     2498           
  Branches        315      315           
=========================================
  Hits           1665     1665           
  Misses          710      710           
  Partials        123      123           

Impacted Files	Coverage Δ	Complexity Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	77.77% <ø> (ø)	20.00 <0.00> (ø)
.../main/java/seedu/address/model/IngredientBook.java	88.88% <100.00%> (ø)	12.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	84.44% <100.00%> (ø)	37.00 <1.00> (ø)
src/main/java/seedu/address/model/Model.java	40.74% <0.00%> (ø)	5.00% <0.00%> (ø%)

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[ureshiiYing]

I realised I did not review your DG, so I have done it now.

[yanbingtao]

LGTM

[Persdre]

LGTM