Improve missing header message

See gh-103

Author: philwebb

spring-javaformat/spring-javaformat-checkstyle/src/main/resources/io/spring/javaformat/checkstyle/check/messages.properties

@@ -1,5 +1,6 @@
 catch.singleLetter=Single letter catch variable (use "ex" instead).
 header.unexpected=Unexpected header.
+header.mismatch=Line does not match expected header line of ''{0}''.
 javadoc.badCase=Javadoc element descriptions should not start with an uppercase letter.
 javadoc.missingSince=Missing Javadoc @since tag.
 javadoc.publicSince=Javadoc @since tag should not be used on private classes.

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/HeaderDate.txt

@@ -1 +1,3 @@
-+HeaderDate.java:2: header.mismatch [SpringHeader]
++HeaderDate.java:2
++Line does not match expected header
++[SpringHeader]

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/HeaderMismatch.txt

@@ -1 +1,3 @@
-+HeaderMismatch.java:5: header.mismatch [SpringHeader]
++HeaderMismatch.java:5
++Line does not match expected header
++[SpringHeader]

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/HeaderTooLong.txt

@@ -1 +1,3 @@
-+HeaderTooLong.java:15: header.mismatch [SpringHeader]
++HeaderTooLong.java:15:
++Line does not match expected header
++[SpringHeader]

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/HeaderTooShort.txt

@@ -1 +1,3 @@
-+HeaderTooShort.java:14: header.mismatch [SpringHeader]
++HeaderTooShort.java:14
++Line does not match expected header
++[SpringHeader]