Refine searchmode

[cshao02]

Fixes #177 #169

Allows exiting of searchmode with "exit"
Changed commant to exit searchmode from "exitsearchmode" to "exitsearch"
Changed error msg from "Unknown Command" to display that the person is in command mode.

[brianchoon]

LGTM!

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../java/seedu/address/model/util/SampleDataUtil.java	0.00%	6 Missing ⚠️
.../seedu/address/logic/parser/AddressBookParser.java	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <100.00%> (ø)	4.00 <2.00> (+2.00)
...ogic/commands/event/commands/ViewEventCommand.java	78.94% <ø> (ø)	6.00 <0.00> (ø)
...gic/commands/searchmode/ExitSearchModeCommand.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	100.00% <ø> (ø)	33.00 <0.00> (ø)
src/main/java/seedu/address/model/event/Event.java	91.50% <ø> (ø)	45.00 <0.00> (ø)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
.../seedu/address/logic/parser/AddressBookParser.java	87.50% <50.00%> (+0.32%)	21.00 <0.00> (+1.00)
.../java/seedu/address/model/util/SampleDataUtil.java	21.05% <0.00%> (ø)	3.00 <0.00> (ø)