Make assertion messages in CookieCsrfTokenRepository clearer

Changes assertion message format from 'X is not null' to
'X cannot be null' since this is more meaningful when the error
occurs and the message is printed in the logs.

Closes gh-9195

Author: sedran

web/src/main/java/org/springframework/security/web/csrf/CookieCsrfTokenRepository.java

@@ -100,7 +100,7 @@ public CsrfToken loadToken(HttpServletRequest request) {
 	 * provide a token
 	 */
 	public void setParameterName(String parameterName) {
-		Assert.notNull(parameterName, "parameterName is not null");
+		Assert.notNull(parameterName, "parameterName cannot be null");
 		this.parameterName = parameterName;
 	}
 
@@ -110,7 +110,7 @@ public void setParameterName(String parameterName) {
 	 * token
 	 */
 	public void setHeaderName(String headerName) {
-		Assert.notNull(headerName, "headerName is not null");
+		Assert.notNull(headerName, "headerName cannot be null");
 		this.headerName = headerName;
 	}
 
@@ -120,7 +120,7 @@ public void setHeaderName(String headerName) {
 	 * and read from
 	 */
 	public void setCookieName(String cookieName) {
-		Assert.notNull(cookieName, "cookieName is not null");
+		Assert.notNull(cookieName, "cookieName cannot be null");
 		this.cookieName = cookieName;
 	}
 
@@ -216,7 +216,7 @@ public void setSecure(Boolean secure) {
 	 * @since 5.5
 	 */
 	public void setCookieMaxAge(int cookieMaxAge) {
-		Assert.isTrue(cookieMaxAge != 0, "cookieMaxAge is not zero");
+		Assert.isTrue(cookieMaxAge != 0, "cookieMaxAge cannot be zero");
 		this.cookieMaxAge = cookieMaxAge;
 	}