Raise an exception when timestamp is malformed

Previously, when a provided timestamp didn't match the regex format, it
would be configured as `nil`. This was nonsensical since a `nil`
timestamp is unusable, which allowed for subtle bugs to creep in.

Instead, we should alert the user as soon as an improper timestamp is
encountered by raising an informative exception.

The documentation has also been updated to reflect the expectations for
tiemstamp format.

Fixes #140.

README.md

@@ -68,6 +68,8 @@ Biz.configure do |config|
 end
 ```
 
+Configured timestamps must be in either `HH:MM` or `HH:MM:SS` format.
+
 Shifts act as exceptions to the hours configured for a particular date; that is,
 if a date is configured with both hours-based intervals and shifts, the shifts
 are in force and the intervals are disregarded.

lib/biz/day_time.rb

@@ -38,7 +38,10 @@ def from_timestamp(timestamp)
               match[:minute].to_i * Time.minute_seconds +
               match[:second].to_i
           )
-        }
+        } or fail(
+          Error::Configuration,
+          'invalid timestamp: must be in `HH:MM` or `HH:MM:SS` format'
+        )
       end
 
       def midnight

spec/day_time_spec.rb

@@ -78,33 +78,63 @@
   end
 
   describe '.from_timestamp' do
-    context 'when the timestamp is malformed' do
+    context 'when the timestamp is not a timestamp' do
       let(:timestamp) { 'timestamp' }
 
-      it 'returns nil' do
-        expect(described_class.from_timestamp(timestamp)).to eq nil
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
+      end
+    end
+
+    context 'when the timestamp is in `H:MM` format' do
+      let(:timestamp) { '5:35' }
+
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
       end
     end
 
-    context 'when the timestamp is well formed' do
-      context 'without seconds' do
-        let(:timestamp) { '21:43' }
+    context 'when the timestamp is in `HH:M` format' do
+      let(:timestamp) { '12:3' }
 
-        it 'returns the appropriate day time' do
-          expect(described_class.from_timestamp(timestamp)).to eq(
-            described_class.new(day_second(hour: 21, min: 43))
-          )
-        end
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
       end
+    end
 
-      context 'with seconds' do
-        let(:timestamp) { '10:55:23' }
+    context 'when the timestamp is in `HH:MM:S` format' do
+      let(:timestamp) { '11:35:3' }
 
-        it 'returns the appropriate day time' do
-          expect(described_class.from_timestamp(timestamp)).to eq(
-            described_class.new(day_second(hour: 10, min: 55, sec: 23))
-          )
-        end
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
+      end
+    end
+
+    context 'when the timestamp is in `HH:MM` format' do
+      let(:timestamp) { '21:43' }
+
+      it 'returns the appropriate day time' do
+        expect(described_class.from_timestamp(timestamp)).to eq(
+          described_class.new(day_second(hour: 21, min: 43))
+        )
+      end
+    end
+
+    context 'when the timestamp is in `HH:MM:SS` format' do
+      let(:timestamp) { '10:55:23' }
+
+      it 'returns the appropriate day time' do
+        expect(described_class.from_timestamp(timestamp)).to eq(
+          described_class.new(day_second(hour: 10, min: 55, sec: 23))
+        )
       end
     end
   end